Unless she flat out tells you that she's interested, there's no "sure sign", however there are small signs that you can look for.
Does she seem to go out of her way to talk to you or be around you?
When you tell a joke, does she laugh at it even when others don't?
If you're talking to her and the conversation hangs, does she re-initiate it?
Do you notice her standing near you even when her friends are around?
Does she steal glances at you?
Obviously there are more, and the more signals she sends, the better. If you think she likes you then don't be afraid to make a move. If you wait too long she might misinterpret it as a rejection, or she might be put off by your lack of confidence.