I include stuff like this in Medicine checks, just like I include magical locks and trap elements in use of Thieve's Tools. It's a world full of magic.I'd also allow the use of Religion as a sort of "Arcana for the divine and profane".
Obviously Arcana could work.
I'd not wait for the players to say, "hey can I check her out for magical badness?", and instead just tell the player with the highest passive score in one of those that it occurs to them that something is off here more than simple behavior, and ask for a check.
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