Eyes...
It's in the eyes. You can tell if a girl really wants you by the way they look at you. Hell, guys do it to them all the time. If she looks at you, the glances will be brief for the most part (in my experience). But if she keeps looking that means she wants you to talk to her.
If you get the multiple glace (you'll know it when it happens) go over and say "hello". Simple as that. May not go any further than that, but at least you made and effort. And effort is very important to women (see confidence above).
Even if she's not interested odds are she'll still talk with you, and by that point you better know if it's going well or not. Otherwise your on the road to rejection. Just have fun, and for the most part....be respectful.
'cak