Can I use OOC information? Frankly, there's not a lot of OOC info that Jareth won't divulge on his own. She's more than free to deduce that he has an undefinable thing for young ladies (it will become obvious) and that, although he might talk big and have impressive magical ability, he can't actually do wicked things with that magic.

Is there anything in particular you would like Shirley to notice? She might notice that any tricks Jareth does (conjuring crystals and the like) isn't slight of hand. Most everything about him is either incredibly obvious or hopelessly and impossibly vague (is he fey? what are the limits of his magic?).

Is there anything you specifically don't want her to notice? Nnnope. She can notice him using magic, turning into a white barn owl, swishing about dramatically, creeping on young ladies...