Like, in crowded situations, or when the two of you happen to be standing or sitting close together, and her legs come to brush yours and stay there, should you see that as a sign of interest? This is in situations where you can't talk easily (like in class or she's with friends on the bus), so what you have to go on (at least at first) is just body language, etc.

I don't want to read into something that's probably innocent and accidental, but if it just happens on its own (meaning they were the ones that moved and made contact), and if they don't move away (so you're both still touching), that has to be done at least somewhat purposefully, right? It's hard to tell because if it's crowded, any slight movement will make you brush up against someone.