I would say most of the time, there are (obvious) signals, especially if you do not know the girl or anyone who knows her. However I remember knowing a girl (who had a boyfriend) who I met a couple of times at a club and stuff who was always with some female friend who didn't seem interested in me at all next thing I know (to be honest I'm wondering how this all went down..especially the 'transition' part I'm kissing that female friend of hers every other weekend and end up in bed with her and then break it off, because she had a crush on me...I also know a girl who never really acted like she was into my friends..like I couldnt have told it at all...and little time later she has the biggest crush on him ever...I think sometimes a girl might be scared to show her lust/affection for a guy if they have mutual friends or know each other already..if a girl is interested and you dont know her at all, even if she's sorta shy there's probably always gonna be some obvious glances at least...