Most Helpful Guy
There are so many clues you just have to put your mind to it and you are set to go.
Eye contact is a big thing, if she's always watching you, there's good chances she's into you, especially if she is talking to other guys just to see if you are looking at her
Also conversation, if she's always find your stupid jokes funny and she is getting close to you while talking, grabing your arm, it's a set deal.
Finally, you can look at her pupil. The pupil tend to dilate when a girl is interested in a guy so thats a pretty usefull way.
And also just approach her and get close, if she doesn't back off that much, make whatever move you want.