The only way to know how someone identifies in terms of their orientation is to ask them. Obviously, this wouldn’t be appropriate if you didn’t know the person. Each individual has the right to define their own identity, sexual or otherwise.
Lots of times, we judge people based on stereotypes. So we might assume, for example, that a gay man is going to act feminine and a lesbian is going to act masculine. These are really examples of people not conforming to rigid gender expectations. Because of these stereotypes, teens who don’t conform to their gender’s stereotypes are often perceived to be gay or lesbian and face the same harassment as people who are.