How do I know if someone else is bisexual?

The short answer is that you probably won't know until someone tells you - you can't tell whether a man or woman or a transgendered person is bisexual just by looking at them. Bisexual people come in all shapes and sizes. You cannot tell a bisexual person by the gender of their partner or the friends they keep. It is important to remember that if you identify as bisexual, how you look is up to you, just the same as if you are heterosexual, gay or lesbian. How you dress and how you behave is about your personal identity, not a stereotype.