... or you could walk up them and show them symbols or something related to magick and ask them if they know what they are and if they don't know just tell you had a mythology test(this will only work if they're not in any of your classes.
i dont think there a sure fire way to tell without asking. you might have just picked up something that was inadvertently projecting. that sorta things happen sometimes. the best example of this is i can think of at the time is some one thing thinking a smidge loud in their head and then you pick up on it.