For me, sometimes I think I know, but most of the time I'm not sure. If they're giving you a signal that they like you, and you don't notice, obviously you're not interested. For the one you're pursuing, play it safe, and if you don't get any signs, then try again. To know for sure, be more direct to the point where you're almost confessing. It would be like a moment of truth sort of thing. If it is clear that she dislikes you, give up. Or you could always try something more outrageous to get her to like you.