Don't overthink it. If she doesn't like you she won't let you invade her personal space, she'll move her head back so that your faces don't get close enough, or she'll take a step back. If she likes you she will stand right next to you and she won't move her head away.
Here's a video from Keys To The VIP that I think demonstrates the right way to build sexual tension and go for the kiss.
http://www.youtube.com/watch?v=A5yIOTKt_so
The second guy is the best one to watch since he uses triangular gazing, but if you watch both of them you'll see exactly what I'm saying - if the girl will let you invade her personal space and get close she will let you kiss her.