How to prove someone is gay?

You can't prove they are gay. It is something only they truly know and is something you can't figure out for them. If you attempt to prove they are gay and they are not ready to say anything if they are gay, you are only going to push them away from you and you are going to lose that person as a friend. What is more important; knowing their uality even if you are wrong, or being a real friend to them?