from my experience, when a girl has really been into me. She will give me consistent eye contact, she will try to start a conversation. I've also had a girl put her hand on my leg at the bar and rub it while she talked to me, but she was drunk and probably would have done it to anyone lol. When I'm interested in a girl, i also give her eye contact, and i will try to start a conversation(if im not nervous at the time) There's also other signs like the aforementioned dilation of the pupils (i've yet to have seen that up close, but I'm bound to see it at some point) and their feet will be pointing towards you, not away because you are the center of their attention. I've also noticed women who had no real interest, but momentary curiosity give a "look of approval" which i think is more of a naturally triggered response than anything else. They're looking for a mate, so they will look at whatever catches their eye, even if they have no intent on following through with hooking up with that person. People think that attraction is merely a sight based feeling that builds on the surface, but there is an internal animal science behind it too.