The thing I look for are called indicators of interest (IOI). They can run the gamut, the most common are open body language (meaning the girl has her body facing towards you and is not sideways or giving you the shoulder), strong eye contact (or conversely, the girl is staring at your toes and giggles every time she looks at your eyes), physical contact of any kind (this one is huge), any kind of inuendos the girl might make. These are the biggest ones. With enough reference experiences, picking up on these will be a breeze. At this point, I can tell just how much a girl who is attracted to me at pretty much any point, with high accuracy.