It depends on the girl. It's said to be a classic sign of interest, but taken alone it could be anything. One of the best indicators is dilated pupils, presuming you can see the difference. But honestly, all that stuff is not actually that useful in the end, because the more time you spend trying to figure out if someone likes you, the less time you could be spending with them as a person - which usually ends up with them like you more than someone with a clipboard taking notes.