It's hard to say, but I have noticed that bi girls tend to have one of those crown cutz where their hair is shaved around the sides you know. Shaved heads are a decent indication- but this stuff is not full proof. I'm of the opinion that you can tell a lesbian is a lesbian if she looks like a bull dog right?
If you're talking about lipstick lesbians well they're just people, sorry. You have to ask them for that type of information.