Of course there are the obvious one--eye contact, hair playing, finding excuses to talk to you, etc. But there's a couple that you might mistake for negative interest.
For me the #1 sign of a girl's sexual interest is that she gets offended/pissed/frustrated at something I say really early on in the interaction. Seems a little counter-intuitive, but it's indicative of heavy investment before she even knows who you are--to the extent that her reaction is totally disproportional to the situation (often harmless, maybe ambiguous remarks). It's kind of a case where she throws the whole **** test upfront (she's subconsciously giving you an opportunity to provide her with a rapid DOWN/UP emotional experience where the only way to totally diffuse the situation is sex).
Another one that guys might confuse with disinterest is when a girl is visibly conflicted--maybe even avoiding you or a situation where the two of you might end up alone, avoiding eye contact, or becoming really quiet. Basically making every effort to put on the breaks because she knows that if she puts herself in that position she won't be able to stop herself from being carried away by her natural desires.
But really you should focus more on if she's doing enough to interest you instead of figuring out her levels of interest. In a lot of cases, dudes are approaching too late (waiting on signs from her) and bailing too early (mistaking more complex emotional reactions for disinterest), limiting themselves to only the really obvious lays.