Firstly its not a science and people can get their wires crossed..a girl just may catch your eye by sheer chance but you, the young stallion that you are, might think she’s ‘clocking me she is’ and take that as a green light…
My advice is if you want to be as full proof as possible try and gage as many signals as possible.
The well known signs are eye contact (including looking away and looking back, watch out though if they do this consistently it may just be a nervous twitch), smiling (beware of sympathy smiles though they are the complete opposite!!), pays you here full attention (unless your a teacher of some sort then she is probably just trying to learn from you so keep it professional), is tactile (again judge the circumstances ..if she is fitting you in a suit she is going to need to touch you! ), teasing or being sarcastic to you (especially true of shyer girls) but again a pure insult such as ‘f off you smelly, short , pissed , lecherous looser” probably means she isnt interested .. But conversely they could still be interested if they ignore you!! yes they could be waiting to see if you are interested and also if you have the balls to make a move..
Yes women are complicated and there are hundreds of books out there on the subject (and I will have a chapter on it in my forthcoming book). But my take on it is this. If you see a girl you fancy make a move..end of..(how many girls have I interviewed and loads of stunners too who say a) they would never make the first move and b) most guys don’t make the move.. As they say you’ve got to be in it to win it. So if you get any of the tell tell signals and even if you don’t and just pass a girl on the street and think core I’d love a bit of that..do something about it..ask for her number or out for a drink. But don’t be sleazy about it saying what nice knockers they are or how slappable her rear end is that generally doesn’t go down well (but in some places it might it all depends..) oh and do take no for an answer .. 97% of girls will be flattered.. the other 3% well..anything could happen..