Simple answer is: You don't. The only way you know for sure that someone is a lesbian is if they tell you they are, or are open about it.
That is up to you. Might be better to wait until you know though. If you feel you have to tell someone. Regardless of what you are - you are.
tell him the reasons why you are a lesbian...he will understand.
If you mean recognize that they are lesbians, the men cannot do that without asking them.
Lesbians have ordinary names, usually. You can't tell whether someone's a lesbian by their name. Just name your lesbian character whatever you want, and make her a realistic person.
You can tell if you are a lesbian if you are attracted exclusively to other women.
Their age is still young and they are still growing and learning about them self. If they are lesbian then maybe someday they will tell you and it would be appropriate for you to accept them.
to tell someone your lesbain like your parents or friends all you have to do is be brave and think that they will still love you even if you are a lesbain
Trust your gay-dar. If the responses to your flirting are positive there is a high probability.
Talk to someone and tell them that they are ugly. If it is a girl say you are lesbian and if a boy say you are gay.
If you are a curious and need to tell if you are a lesbian, think about a woman's body. If it turns you on in a sexual way, you are likely a lesbian.
You can't make someone else a lesbian. A woman who previously identified as straight may come out, or make an exception for a specific person, but there is no way to turn someone into a lesbian.