It's no different from straight people... You hang out with someone, and if you enjoy it, you hang out with them more. After a few weeks of that, you try to get the sense if they're hanging out with other people or if you're (willingly, happily) taking up more and more of their time.
Then, after a maybe a month or two of seeing each other regularly, you might broach the subject in a casual way: "hey, so I was going to mention you to a friend of mine and I wasn't sure how to refer to you... are you my boyfriend? Are we dating?"
That's a good way to not overload the moment with expectations or pressures. If he hesitates, then you can always quickly add, "..It's fine either way, I'm enjoying our time together regardless. It would just be nice to know what to call it..." and smile sweetly.
If he does indeed recoil, or hesitates, or demurs, or just looks stricken, then you will have an answer in that too, in that he's maybe not ready to call it anything more than friends/fuckbuddies. Then it's up to you to determine if you are enjoying the status quo, do you want more, are you willing to wait any longer or do you want to set a time limit, etc?
I think 2 or 3 months of serially seeing someone is the point at which, if either of you wants something more serious, you should start defining what's happening. And if neither of you wants anything more serious, then you should use your words and probably say that too so that you both know that you're just there until something better comes along.