How do magi identify each other? It's illegal to scry on members of the order of hermes; so how do you detect if one is present?
The usual means to identify a maga or a redcap is the ArM5 p.86 voting sigil. Just how that works in detail is tmk nowhere described. Wouldn't this topic make a nice subrosa article some time?
it makes sense to assume, that a maga is at least an Arcane Connection to her sigil: so if she is present it can be verified that a sigil is hers.
It must be possible to identify - likely by easy magic - an item as a voting sigil, even if its owner is not present: after all, another magus can vote with a maga's sigil as her proxy, if he holds it at Tribunal.
Those Tremere, whose voting sigils are held by their parentes, might have similar signs identifying them, that are not valid for voting at Tribunals.
Cheers
The population of the Order of Hermes is around 1500 magi, give or take, if I remember correctly. Similar to a village where everyone knows each other. Or at least knows each other heads of families. As in those villages, one would ask "You're the daughter of...?", in the OoH, one would ask "You're the filia of...?". So in my opinion, most magi know most of the order.
Also, keep in mind that the use of magic to disguise oneself, if one gains any information on another magus during the disguise, constitute scrying.
In our sagas, magi tend to display who they are. If you want something more subtle, you can have them share "a secret handshake".
I think I know the kind of thing which Hroppa has in mind as it came up in our game yesterday...
IMO magi know each other more along the lines of the aristocracy of Europe than people who live in the same village - by name and reputation, and if not by that then by lineage (parens, etc) and affiliation (covenant and house, and perhaps society or cult). But they don't necessarily know each other by sight - there are meeting of regional tribunals, but you are unlikely to meet most magi of other tribunals, so you would not recognise the majority of members of the Order if you happened to pass them on the street.
And as a concrete example which came up yesterday, two magi with the Gentle Gift who have never previously seen each other are in the same hall. Neither have been publicly declared to those present as a member of the Order. One of the magi discreetly casts a Mentem spell or two on individuals present and happens to cast a spell on the other magus, not knowing (or so he says) that he is a magus. The spell is blocked by Parma Magica, and so the caster can then guess what has happened, and the target suspects an attempted magical attack of some sort. Would this count as a breach of the code, if done in ignorance?
David
In general, no. A spell cast with zero penetration that only reveals that "hey, this guy is a magus" is not going to be prosecuted.
On the other hand, a spell that accidentally pries into the secrets of a Redcap is illegal. If the magus can convince everyone that it was an accident, then he's still committed a crime but is only liable for damages. (On the other hand, a Redcap is usually fairly obvious on account of the big red hat.)
As for identifying a magus, Jerbiton has invented an outfit that magi can wear if they want to display themselves as magi to mundanes - most magi don't wear it.
A magus may be
Dressed in an expensive, strange outfit that does not mark them as a nobleman or a wealthy merchant.
Dressed in something strange, possibly classical or extremely foreign despite not apparently being a foreigner.
Wearing something with a Hermetic symbol, or several symbols, often of their House.
Wearing more bright colors than any ordinary mortal.
Avoided by a crowd, or subject to rudeness by others for no obvious reason. (Magi do not directly sense the strangeness of the Gift, at least not if they have an active Parma Magica.)
Sporting a bizarre hairstyle.
Attended by armed guards and/or odd assistants.
Speaking Hermetic Latin, which is not quite the same as Church Latin.
Treated as an important person, despite not being an obviously wealthy person, and possibly having no grasp of mortal manners.
Possessed of an otherworldly air or attitude, aside from the effect of the Gift (call it the Affect of the Gifted?).
Casually and prominently displaying an elaborate, odd, or intricately worked staff or other item.
In the company of an unusual animal, or an ordinary animal that seems exceptionally smart and well trained.
Criamon, Flambeau, and Redcaps are generally easy to spot. Some Tytalus magi wear green or purple. Marinita often stand out in a crowd. Verditius are harder to spot, but may dress like a strange cross between a merchant and a craftsman with too much money.
Of course, to some magi, the mage is easy to spot - he's the one with sensible clothing and a proper attitude.
Grogs and companions may have an easier time spotting the strange and suspicious character - oh, a mage. Careful...
I have some doubts about scrying. I'll assume a resisted Intellego spell casts a black silouette, but not sure of that. Would appreciate some enlightenment.
A magus is disguised in peasant robes in a crowd.
Another magus approaches and see something strange in that peasant. Casts InCo with 0 pen.
Disguised magus appears as a black silouette.
Who would win a scrying case at Tribunal?
For starters, I'd generally disagree with this. All that happens with a successfully cast but resisted Intellego spell is you don't get any information.
One magus used Intellego magic to attempt to discern information about another magus. The other magus was nonmagically disguising themselves in peasant attire. Which person is using magic to scry should be abundantly obvious. (Note that intelligence gathering isn't inherently scrying - it's only scrying if magic is used to do it. So the peasant-dressed magus couldn't be accused of scrying even if he was actively spying on the other magus, never mind that he's minding his own business here.)
Well, I'm restarting a non-canonical saga where magi are a part of secular power in Europe.
So, magi do wear a distinctive symbol (the pentagram) as a display of them being magi. Simple.
And, well, I discarded for the most part the rule against scrying. IMS, scrying is only a crime if done againt an identified magus, supposedly with the intent to discover something about him, so there is no inadvertent scrying at all.
(Tribunals use a teleological interpretation of rules, as I do)
Scrying is allowed to quaesitors and also if the magus could prove that he intended to help (or acts for the good of the scried magus, for instance an In Co spell to know what's wrong, or to rescue a disappeared sodales).
See Houses of Hermes:True Lineages page 52 under Scrying - "When using scrying spells in mundane society cautious magi usually cast without Penetration (see Forceless Casting insert). In this way even if a magus is accidentally caught within a target, his privacy is usually protected. However, this is not absolutely safe." (and goes on to discuss what happens in the case of redcaps)...and at the end of that paragraph "If there was no intent or secrets revealed the case is trivial"
So in the case of the mundanely disguised wizard being hit with a forceless casting Intellego spell, usually if the detected wizard tries to complain their case will be dismissed as a waste of time. If the wizard bringing the complaint is incredibly powerful and has a lot of sway at Tribunal, then the wizard who scried had better make a public apology and offer token damages.
If wizards go out of their way to disguise themselves, they can cause trouble for their sodales - indeed, in Tales of Power the scenario set in the Papal election has a plotline involving a disguised Hermetic magus.
Also I prefer it this way for the moment, an inminent saga aproaching.
I see your point about who used magic to scry, but if the peasant-magus were magically disguised? I guess it's up to Tribunal common sense/power games like Darkwing pointed (thank you both ^^).
Obviously, when you meet suspicious peasants, you don't InCo them. You discreetly (no voice or gestures) MuIm them, with penetration 0, so that if the spell penetrates they briefly and very, very subtly change their appearance - say, they smell subtly of grass and flowers.
If the peasants take on a pleasant scent ... well, they are peasants! Or very devious magi. Or very devious other ... devious ... things.
If the peasants don't take on a pleasant scent ... they are magi in disguise! Or magic resisting critters of some type.
If the magi-in-disguise take you to Tribunal for scrying on them ... you deny all charges, and press a counterclaim against them for scrying on you, as follows. You point out that they were so very shoddy in their disguise that you immediately recognized they were magi. However, they had so very clearly exaggerated with their "peasants have poor hygiene" cliche in their shoddy attempt at disguise, that you needed to refresh the air a little so as to be able to breathe. Hence, the MuIm spell. But the magi in-(shoddy)-disguise had their Parma up! So they noticed your spellcasting by resisting it -- something they would not have otherwise noticed! So it's very clear that you did not scry upon them, but they scried on you, through their Parma, albeit perhaps inadvertently. Just perhaps though -- which peasant has a Parma up? If they simply wanted to disguise themselves as peasants, they should not have kept their Parma up. No, theirs was probably an attempt to deviously gather information on the presence of magi by inducing them to cast odour-masking spells (fair, since it was done deviously but without magic) and detecting those spells through their Parma -- ah, the magic of Bonisagus, his greatest gift to the Order, used to break the Oath! Well, inadvertent or not, theirs was a crime, but in the spirit of fairness that should inspire every magus you only ask for token punishment to be meted on them: a rook of vis -- not for yourself, no! For the Tribunal coffers.
I love playing Tytalus magi 
Also they were depriving you of your magical power by smelling so bad you could'nt cast anything than that MuIm spell! >:(
Keep in mind that reading minds of non-magi may constitute scrying, if they know something about specific magi, so be carefull with the mind reading as well.
It is not against the code to for a magi to disguise themselves non-magically and spy on other magi. The Oath says "I will not use magic to scary upon members of the Order nor shall I use it the to peer into their affairs."
IIRC, there is a quaesitor in the Greater Alps who gathers info in just this manner.
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It always seemed to me that unless a magus has the Gentle Gift virtue, he has a certain mein about him that makes mundanes uncomfortable; maybe it's that he exudes vis, or perhaps it's the way he just holds himself, or the way he dresses, or his lack of peasant stench, or his unnaturally well groomed appearance, or his foreign mannerisms, or his odd manner of speech, or his non-peasant/noble/merchant-like bearing/garb/affectations, or some combination of these.
Whatever this je ne sais quoi is, other magi also perceive it, but aren't weirded out by it the way mundanes are...in fact, quite the contrary.
Anyway, that's the way I run it.