It doesn't come to her immediately; there's a lot about the supernatural world that she doesn't know well enough to reach for instinctively, and most of what she knows is learned by experience. This isn't an experience she already has...but she is a bright girl, and there are only so many places that this can go.

“Wh-- oh.” She frowns, worrying her fingers with her teeth - she'll draw blood like that if she isn't careful, and probably the last thing they need is to literally put blood in the water - and studies him like she's trying to decide whether or not she thinks he's physically up to it. That is, in fact, what she's trying to judge; she quickly comes to the conclusion that she is the worst judge of that and that it's pointless. “Do you think that would work?”