Hey - I thought that magi could instinctively detect changes to aura levels, when they step through them. (ie, without the use of InVi.) If so, does anyone know where this is written down? I'm not finding it in the books anywhere.
ArM5 p.182ff Realms has descriptions of the various auras, and how the impressions they give vary by strength. So a SG can rely on these descriptions to give players hints on the presence and strength of auras.
Sometimes these ArM5 descriptions are quite definite, like:
Cheers
Keep in mind that it is not a full proof method.
Aura influence fauna and flora, temper and mood. For an experience traveler, it is sign that he will use to identify the presence of an aura. Per + Realm Lore could allow to "guestimate" if a person is in an aura or not.
Per + Survival could help to identify that something is different, supernatural.
The difficulty will depends on the aura level.
But a magus without those specific Realm skills has to rely on InVi to detect an aura. The Gift itself is not enough to detect an aura. Casting a spell in an aura could also reveal that magic is more or less difficult to practice, indicating an aura and its type of influence (positive, negative), and a magus could infer what type of aura it is.
(Realm) Stigmatic would help a lot in identification, and Susceptibility to Faerie/Infernal Power could help.
Well, admittedly, the easiest way is to cast a spell and ask the GM what the aura bonus is. 
I mean, If you have a /5 spont spell that you can't QUITE cast, then a change in the aura would also tell.
Finally, if you can explicitly not use all your ability in a TeFo, then you could also dial THAT up and down on a /5 spont, and figure out what the bonus is. However, I don't know if that's allowed.