Wait for another archmagus to challenge him, and observe what happens. Another archmagus is bound to challenge him eventually, if none does, that should mean something.
Ask him to cast the covenant's Aegis of the Hearth. If he does what magnitude is it, does he use any unusual casting patterns.
If he refuses, what is his reason? (and challenge him to Certamen to force him to --- and see if he does actually fight the Certamen).
" Unusual casting patterns" - I'm curious - what angle are you pursuing?
Carrot + stick for me as well. I would make this guy go out discretely ffrom time to time, and maybe some covenfolk notice. They might or might not tell it to the younger magi.
Him making unusual requests for materials would also ring an alarm bell for the magi. Like wanting a virgin's blood, a seed from Ygdrassil or the heart of a faerie.
If the players do not know that there is something weird going on, they might decide to jsut hang around and wait. After all he will avert the impending doom. Or so he said. If he needed any help he would have asked for it. In the meantime you get hermetic prestige and quiet for you to exploit without problems and zero responsibility. Quite a nice suituation: your mystery story is a background feature with no real impact on the covenant anymore.
Remember CH's suggestion: carrot and stick. otherwise they will not bite it.
On the question: What did he do to gain archmage status?
Cheers,
Xavi
- Does he ever cast spells with longer-than-Momentary duration on an object or location other than himself?
- Assuming the answer is yes: Use Intellego Vim magic to probe the object or location. Is the spell truly of Hermetic origin? Does it match the expected Technique+Form combination (as opposed to, say, Creo or Rego Imaginem)?
- Assuming the answer is yes: From ArM, p 86: "All kinds of sigil leave some traces behind in the magic, and skilled Hermetic investigators can use this sigil to find out who cast a spell." Can it be verified that this particular magus is the caster of the spell he seems to be casting?
Who, or what faction, does he most commonly express displeasure, enmity, or hatred towards? (ex: house Merinita, the Church, demons, the Iberian Tribunal, etc?)
In private, use Creo Aquam with Sun duration to create some wine in a goblet. Join this magus for dinner, or otherwise encounter him, and accidentally spill the magically created liquid on him. Is it repelled by his Parma Magica?
If he does everything we are suggesting, the quaesitor and his ssodales will gain quite a reputation for paranoid behaviour 
CHeers,
Xavi
If I engage the archmage in "small talk" and pump him for information about his exploits, what topics does he focus on? Which ones does he seem to avoid?
e.g. does he talk about his own accomplishments? Those of others? Does he talk about Notatus at all? Does he go into lengthy descriptions of spell-craft or avoid it? "And, at that moment, Notatus unleashed a magnificent Muto Auram spell, transforming all the air around into molten silver...of course, such a thing wouldn't have been possible without his extensive knowledge of...." yada yada yada
Given he is the oldest Bonisagus magus in the Tribunal, is he the Praeco/Primus of the House ? Does he claim the position ? Does someone want/suggest him to do so ?
(Given that Ezz hasn't posted here in 2 weeks, I'm guessing he got the answer he was looking for.
He wasn't presenting this as a game, a puzzle that would we would eventually solve and "win" - he was specifically asking for help to see how fast we got close. I'm guessing either we clearly did or clearly didn't, and he got his answer.
Game over. 
)
Someone is saying he knows about something dooming the tribunal or the Order!
My god !
He's he a fraud, might we ask ourself? of course, it's possible.
But first,
What can we understand of it ?
Such a strange thing to think, that no ones thought of asking this question first. Why not try to fill the gap between what is going on in a Players' mind and a Magus' mind, and call this sort activity roleplaying? 
Trying to check whether he is using Faerie magic, some sort of crutch like Verditius casting runes, or something more exotic to cast spells with. These are things that he probably wouldn't be doing if he is who he says he is --- but are things that an impostor might require. I think that Ezzelino suggested that he was only casting low magnitude spells, which might be possible to fake. Getting the elder to cast the Aegis, which seems a sensible thing for the young magi to request, is a way to try to force him to cast a high magnitude effect...or explain why he can't.
Ah, mickle crafty - well played! 8)
Well, thanks a lot folks - I am now convinced that the mystery is sufficiently "robust", though a few of you got really close. If you got curious, read on.
The "old bonisagus" character in question is going to be a PC - the companion PC of our beta storyguide (his "magus" character is a mythic companion, so this won't be clear to the other players for a while). Actually, I am writing "old bonisagus" in quotes because there's no bonisagus at all. There's only the cat. The cat is a faerie (perhaps the former familiar of the archmage, or a faerie taking its place), and the archmage is just a sophisticated illusion (a power similar to "The Shadow of Human Life", plus "Unseen Porter" for interacting with solid stuff).
The archmage will ask for an Aegis token for his beloved cat every time he visits a foreign Aegis, though he does not ask one for himself 
And if he disappears when hit by a swordblow, only to reappear later ... "HeHe, an old trick, but handy, isn't it? Much better than invisibility!". I hope this will actually confound the guesses of many of our fellow players. Casting spells is a bit trickier, but the cat has a few extra powers to make it look like the mage can actually cast at least minor magics. Certamen is trickier to manage, but in the Rhine it's going to be much easier.
Cool.
As you mention, he will need a token to get into a foreign Aegis --- which you are right he might be able to claim he needs on behalf of his "cat". The bigger issue will be whether he can participate authentically in the yearly casting of the covenant's Aegis. Even if he isn't the magus casting the Aegis, he would be expected to "participate" in the casting. Participating in the casting of his home Aegis is what makes it "not a foreign Aegis". The RAW are a little bit vague on what "participating" means, but it is possible that if he is not really participating as a magus, it could be noticed by the participating magi.
He may also run into issues if during the course of an adventure another magus decides it is a good idea to cast either a Ward against Faeries or some sort of Room or Structure sized Faerie equivalent of Demon's Eternal Oblivion. Getting stuck behind the ward, or accidentally crippled by the spell would be a bit of a giveaway.
The failure of "friendly" Corpus spells (healing, teleports, etc) might be noticed too.
+1
Totally got me hooked and running down the wrong alley. Good use of misdirection!
I wonder - if the cat was a familiar of a lost magi (not "dead", simply MIA, either this bonisagus or some other mage), and had been taught Magic Theory, could it then "participate" in the Ritual?
I always thought it was basically anyone who was around who had reason to be included - which usually implies anyone with magical talents or supernatural abilities, but doesn't explicitly demand as much.
As a mundane in RL, I've participated in a lot of "rituals" - not exactly sure what it got me, but I was there. 
(And the Corpus spells could be written off by a good Parma.)
Participate, yes. Mundanes and people with supernatural abilities participated on the ritual on a regular basis.... Or used to in previous editions (haven't checked the books). However, IMS magi tend to cast the Aegis with Wixzard's Communion, and here is where the Big Boni will not have the skills to perform. It will be odd if he claims not to know the Communion spell, but weirder things have been seen...
Really cool BTW. I read this concept in a comic a while ago (the suposed "daughter" of one of the characters was also an illusion) and thought it really cool. It has lost none of its appeal 
Grats on a cool story concept.
Cheers,
Xavi
Sure. I believe that the RAW allows hedge wizards to "participate" too. But the rules are vague on what "participating" actually means.
Can the caster of the ritual tell who is participating? Do the participants have to do something mystical? Certainly, something mystical happens (they are not affected by the Aegis).
To use a real life example, think of singing the national anthem at some sporting fixture. Sure you can stand up, and even mouth the words, but if you are not really singing people (near you at least) can tell --- if they care. Is there a mystical equivalent of this for the Aegis? Can the cat fake this?
But the main "caster" of those rituals didn't have genuine mystical powers. (Or if you prefer, they didn't have such practical mystical powers as Hermetic magi have). So they presumably couldn't tell whether you were authentically participating or not.
Except that if a friendly Corpus spell was being used, the caster would expect the target (i.e. the fake magus) to gratefully lower his Parma.
Crotchety, eccentric archmage: "Expect away, cantrip boy!" 8)