How do you know if a girl is bi/?

Ok so I'm 19f and have been straight all my life and am happy with men but am single at the moment. Over the last couple of years I've been wondering if I'm bi or not - I think I am but I guess that's another question. Just wondering how you know if a girl is into me or not and if she's bi/? There's been like 4 or 5 girls in the past month who, on nights out, have been really touchy feely with me. I'm wary that perhaps this is to do with alcohol as I'm like that myself when I'm drunk. But these girls are hugging me, touching me, flirting, looking at me smiling, giving me the eyes etc. If any of them were guys I wouldn't doubt for a second that they were into me. But the fact that they're girls (a few of whom I'm close friends with) makes me unsure. On one occasion, I was sitting with a girl I know in a club and I had my arm around her shoulder and she grabbed my hand and put it on her **** and later she was rubbing my thighs. To complicate this further, she has a boyfiend so I dunno why she was doing that I guess she was just really drunk. Sorry for such a long winded question but what I'm getting at is this; is this sort of behaviour normal for straight girls or does it mean that they like me/want to kiss me/whatever? Cos I don't want to go in and kiss one of them to only have them going like wtf?? Pleaase help I'm so confused! (By the way, I'm not particularly talking about this girl in the club because she's a friend and has a boyfriend so I'd never do that!)