I'm falling for a straight girl. She flirts with me but how do I know if she's bi without asking?

I've known I'm a for a long time now, though no-one knows or even suspects since I'm girly and I tend to get with guys in clubs when I'm drunk. Whenever I really fancy a girl I always get the feeling that they like me back, which is a vibe I never pick up on with boys. Anyway, there's this girl in my hall (I've just started university), and I'm obsessed with her. I think I've deluded myself into thinking that she likes me back, just from the way she acts around me. She's single now but has always had a boyfriend, and they are all the manly macho type. I know most people will say 'just ask her' but I'm scared that this will drive her away, because she's such a good friend too. I know you can't change someone's uality, but I just need to find a way to see if she has ANY in her at all.