Ok so how do you let gay men know your straight right off without being like hey I'm ----- I'm straight nice to meet you (I find that rude for some reason). Because I hate getting into situations where it's oh sorry bro I'm straight and it's always a little awkward after that.
Let's unpack this question.
1. When you say "how do you let gay men know you're straight," do you mean to say, "how do you let gay men know you are straight, and therefore unavailable for sexual/romantic involvement"?
-- if so, this means that you think of yourself as a potential sexual object for the other person. you are demonstrating that you assume he would be sexually interested in you. that might not be the case. what if he has a partner and doesn't find you attractive but just wants to talk to you, because you are interesting and know a lot about...nascar? saying "hey i'm straight" would tend to shut things down fairly quickly.
2. Why would you need to announce your heterosexuality to a gay man right away?
-- given that you have just met someone, perhaps you should try to talk about other topics at the beginning? weather, sports, politics, olymics, big penises, all the standard stuff. saying "hey i'm straight" can be a bit like oversharing.
3. How are you sure that the man you are talking to is gay? What signs/info do you use to judge whether or not he is gay, in your opinion?
-- finally, saying "hey i'm straight" means that you assume the other person you are talking to is gay. no matter what clues you see that make you think he is gay, even if you see him in the middle of sucking dick at gloryholes at a barbara streisand concert, unless he has said so himself it's not cool to label him. You are free to have your own ideas, and you'd probably be right if you saw him manning the gloryholes. But stop yourself before you put a label on him. Who knows, he might just be an effeminate married pastor who enjoys sucking dick on the side.
hope this helps!