How to tell if your man is gay and in the closet
You want to know how to ask your boyfriend if he is a closeted gay man.
Presumably you want to know because you don't want to waste your time with him as a boyfriend if he's really gay. You might still want to keep him as a friend though.
You must suspect that he's really gay or you wouldn't be asking this question. What evidence have you that makes you think that?
Is there something about his demeanor? The way he walks? His speech mannerisms? Something he does with his hands? Is he effeminate in some respect?
These things are notoriously bad and suspect indicators. There are many mincing, straight men. There are few stereotypically gay, gay men. Of the several thousand gay men I've known or met, I can count the number of them on one hand. And they were flamboyant because they wanted to be. It was a deliberate, conscious affectation. They were stereotypically gay by their own design.
Is there some other evidence that causes you to suspect he's gay? Have you seen him in a compromising position with another guy? Does he have gay magazines or literature that you have seen? Have you seen him at a gay hangout? These would be more noteable and trustworthy indicators.
I inquire because it is important that you realize how substantial or insubstantial the evidence is. What you propose asking is an invasive, sensitive, personal question. There ought to be substantial evidence justifying it before you pose it.
Assuming you have such evidence, we reach your question: how do you ask him?
I'm inclined to the direct approach.
Tell him you are serious about your relationship; you need to know that he is too. Tell him that you've found yourself thinking of him in increasingly romantic and emotional ways. If you're old enough and mature enough, and if it's true, tell him you've been thinking about possibly sleeping together. You need to know how he feels about all that.
Tell him someone suggested that he is a closeted gay man and you obviously need to know about that. Be prepared for him to ask who suggested it. Tell him it doesn't matter; you're not going to say. You just need to know the truth.
In all of this, be prepared for him to interrupt with thoughts and questions of his own. You'll be having a conversation not delivering a soliloquy. The conversation may range astray. That's OK. Just return as you can to the substance above. Don't forget where you want the conversation to go.
I said I like the direct approach but I think it's important that you not simply blurt out, "Are you gay?" In the approach above, you set the stage for the ultimate issue. You lead into it. You give him valid reasons why you need to know; you're not just idly curious. At the same time, you are not being accusatory or judgmental. You are coming from a place of good intentions and concern for your relationship, concern for the both of you not just for yourself. This is reasonable and appropriate in the circumstances.
You should also be prepared for him to say he is gay. Know how you want to react should that happen. I suggest you show concern for him and for your relationship. Know in advance whether you want to try to maintain a friendship or breakup all together. Assure him that you will keep his confidence whatever happens between you.