Spidey-Sense: Castiel is an angel. He's a "wavelength of celestial intent", his true form is light and energy. His body is human (and alive, though without human soul inside), but he is very clearly not. If you want, she might even be able to sense the presence of his wings. They're not visible and not tangible, but they're there. She'd likely overall get the impression that he's much, much larger than his body - in the physical and metaphysical sense that this implies. He's just... more than the vessel. Much more.

Demon Essence: Castiel can see the true face of things and look into souls, so he would absolutely be able to see something demonic in her. It means if he wanted to use his offensive abilities on her, it would likely cost him a little more power or maybe be less effective etc. We can hash details for that out whenever they may or may not come up.