I read in a book that a person with deep hate for gays either was once gay or wants to do one. and he always calls gays faggots with a deep hate Is there a way to tell if your uncle was once gay?

First of all, you can't be "once gay". A gay person is gay their entire life. It is possible that your uncle is secretly gay, but just being a bigot and a homophobe isn't enough to go on. He might also be a hateful person in general. There are MANY reasons for hatred of gay people.

Also, there is no way to tell if someone's gay. The only way to know is for him to tell you.