Is your character a magic user/otherwise magical in some way?: Yep.
If yes, can Artemis "sense" this?: Yep.
If yes, is there any specific impression that he would get from them?: His magic comes across as sort of wild, but also there's a creature in his head that he channels it through, a Chimaera. He can probably sense that. Also as with Haruto Souma above, he controls the magic with magical rings and a belt.
Anything else?: The belt eats magic though it hasn't been doing a lot of that lately.