How do you know if your friend is bi or les?

Either just ask them, if they are your friend it should be easy to ask them. Or if not watch them carefully when you are out with them. What gender are they instinctively looking at more? Look at some one of the opposite sex to them and make out how attractive you find them and ask them of their opinion on them. If the answer stutterers to an abnormal amount and is full of unorganised thought/ speech pattens then that's a pretty good indicator. Bi is harder to detect, that requires a lot of watching body language. However it shouldn't matter if they are though. Hope that sort of helps you :)