how do you know if a girl is gay?

Shayla

So say you're out walking down the street, or at a coffee shop and you see someone attractive. and I mean "femme". The "hard to tell" kinds (not that you should assume about anyone), how do you know? I mean yea there's the little things like cut finger nails but that could mean anything. How do you know, and more importantly, if she is, how do you let her know you're interested without making a fool of yourself?