Detecting virtues from casting?

The question has come up a couple times in my campaign if someone could detect certain virtues that pertain to casting like Diedne, Mercurian Magic, Chthonic Magic, etc.. from watching someone cast spells. My initial reaction was that no, you can not detect these things from watching someone cast. Hermetic magic is formalized to the point where it is virtually impossible to distinguish these things, which is why it is easier to teach and whatnot. but after a couple of conversations i'm starting to think that there is a chance to detect these things, even if it is rather slim. Despite Hermetic magic being formalized a little bit of the caster creeps into every spell, his sigil for one, and i would think that spontanious magic would be even more personal as the magus is creating the spell on the fly. Now i havn't exactly decided if i will allow it, but i would put the difficulty in the "Almost Impossible" 24+ difficulty range on a Magic Theory roll, and i would allow an applicable lore ability to add to it. This would STILL make it next to impossible to figure out, but give someone a reason to get a lore skill and make MT more then just a score for a lab total. Lets say i make it 30 for formulaic magic and 24 for spont's.. Even with a 10 MT, 6 in a lore and lets say a perception of 2 you'd need to roll a 12 to detect it! Even spending confidence makes it a 9. Considering none of my players have a 6 in ANY lore (i'd be suprised if any of them have a lore higher then 3) this might as well be an impossible feat for them. I've always been the kind of GM to let someone TRY and do something with an insanely high roll.

I searched the forums and didn't find anything on the topic, so what do you guys think?

In my opinion, you'd have no way of telling between something that is merely a personal variation or something that comes from secret teachings or initiation (i.e. a virtue), unless you already are knowledgeable about those secrets (e.g. have the virtue yourself).

Cthonic Magic is one obvious exception since, when actively used, it involves such amusing things as "invocations to dark gods or sacrifices", which would quickly tell anyone watching that there's something wicked going in.

With Cthonic Magic , there is an invocation component "calling upon dark gods" as one , but not the only option.
Check the description on page 124 in RoP:TI.

Names of Power (page 76 , TMRE) is a Minor Mystery Virtue.
Invocation is part of the Methods & Powers for Divine and Infernal.

Appropriate Lore rolls may be of use in these circumstances at a lower Ease Factor.

I know. I did. I was being fanciful.

@ Fruny ,
my reply was posted before i read yours.
With the forum being a bit unstable (at least on my PC)
i wasn't able to do an edit at the time.

I think a difficulty of 30/24 seems pretty high, even though you're dealing with something very very specific...I think I'd put the difficulty at a 20 for a formulaic, 17 for a spontaneous, and that would just show that there was something in addition to standard Hermetic magic going on...

I think to really get the idea of the sorts of virtues you're suggesting, a magus would need to study the untranslated lab texts of magus with the virtues. Then, with another roll after the season of deciphering, then I think he'd have the chance to know the virtues involved.

Now, whether or not he actually knows what those virtues do, or how they work, or what they're called... that's another matter. ("Hmmm, he seems to draw on some sort of elegant theory here in his rituals. Note how the vis cost is halved! Fascinating, I wonder how he does it?")