A magus believes that a faerie, or faerie magical effect, is altering the appearance of a traveling companion. What is the best way, using hermetic magic, for the magus to detect the effect?
Intelligo Vim, or Intelligo Corpus, depending on the mage's Arts.
Or Intellego Mentem and ask them. (Warning: companion's mind may also have been altered. But if you're good at Mentem you could catch that too.)
Just remember these are realm specific effects so the vanilla effects such SoTAM and SoTTF will not work. You need a Fey version. It can be a level 5, any your companion should not have any resistance unless they have been replaced by a Fey! At that level, you should be able to spont the effect.
Taste of the Faerie influence
You gain the ability for a moment to taste the influence of Fey Magic. If you suspect someone is under their influence, a moment's kiss and you'll know for sure.
InVi 5 R: Per D: Mom T: Taste B5 : Detect active magic (Fey)
If you can afford the extra magnitude, you can make it a touch effect and not be licking kind of magi.
W
There's is also always an InCo 'Who is this person really?"