It's complicated

How To Tell If Someone Is Gay - Foolproof! - YouTube

This might help you

 

Why do you want to know whether he's gay or bi? I sincerely hope you aren't planning on trying to date him if he is. You said he's ten years older than you; 15 and 25 is not only way too wide of an age difference for it to be a healthy relationship, but it's also probably very illegal where you live (I say probably because laws differ in different parts of the world, but in most places, 15 and 25 would be statutory rape). And if you aren't planning on trying to date him, then there's really no reason you need to know whether he's into guys, is there?

 

If you are out to him, you could tell him that you need someone to talk to about issues at school (even if you make them up), how to come out to parents (if you haven't), or anything like that. Why do you want to find out discreetly? Plus, a ten year age gap is a lot considering your own age now.

 

You should talk about it, slowly. This said if you really care about him just remember that he is the one who would have to deal with the justice system, and please don't think that if no one talks about it that it will not become known. I suggest you find someone closer to your age group for a good time and just fantasize about older people if you must. There is nothing wrong with your desire but the consequences of making it real is beyond dangerous for him. Please think about the consequences ...twice

 

Tens years between older adults isn't as significant as ten years between people our age. If one person is 40 and one person is 50, the older person is only 25% older than the other. In your case, this guy is 67% older than you! That's way too big of a difference proportionally. Not to mention the world of awkwardness you'll put him in if you ask.

Look, the vast majority of 25-year-olds don't want to date 15-year-olds, and those who do would only do so because they want to take advantage of you. Any sane adult would say no in this situation. There's no way this situation is going to work out well if you pursue it. It's fine to fantasize, but you really aren't going to gain anything by asking him out. Listen to apliz2005; if you want to ask someone out, find someone in your own age group.

 

Even if he's gay, I really doubt a 25 year old man is going to want to be in a relationship in which he can't have sex with his partner.

And to expand a little on what Owen had to say, you're both in completely different stages of your lives. You're still in high school, a few years from graduating and presumably going to college. Given his age, I assume he's graduated from college already and works now. That's a ma-hassive difference in maturity and life experience. It's sort of akin to the difference between you and, say, a 5 year old. There would still be only 10 years between you, but the way you and a 5 year old think and behave is completely different (and it goes without saying that it would be highly unethical to date a 5 year old). No matter how mature you think you are, you're still a long time away from being independent and getting your life established. The 25 year old is presumably already at that point.

As has been said, it would be better to find someone in your age group to date. That way, if it lasts a long time, you'll have the luxury of moving forward in life and going through the same stages together.