How to know if my friend is gay?

You ask him, or he tells you.

If he tells you.

If he is your friend you should be able to talk seriously.

He'd have to tell you himself, you cannot make that assumption by how he acts and walks or talks, or dresses. Now if he is openly saying some other guy is hot and he'd love to do something him, that'd be a time to ask.

Man that's something vey easy to figure out...why would you have to ask here how to know. Please

Invite him to a swim party /sleep over. While at the local pool, see if he's comfortable showering and changing in front of you or does he keep his swim suit on while showering and change in a stall. At the sleep over, casually bring up the topic of girls and sex, see how he responds and see if he wants to compare penis sizes. If he's not gay, but you still want to experiment, that's perfectly fine especially if you are still going through puberty.

Ask him. If he is and doesn't mind you knowing, he will tell you he is. If he isn't, or is but doesn't want you to know, he will tell you he isn't. You have to accept whatever he tells you, there is no other way of finding out.