Asking someone about their sexual life and sexuality is a very personal issue. Your ability to tread in that area is dependent upon your relationship with that person. If this is someone with whom you are close and with whom you have established a mutual trust, how you approach it will depend on what you know about that person. If you don't have a good, close, trusting relationship with that person, then it is my personal feeling that you don't have the right to be inquiring about it unless it involves your personal health and well being.
Before asking, you might first be sure you have the other person's interest at heart, rather than just a voyeuristic curiosity about the situation. Do you have personal reasons to wonder (like you have been intimate with this person sexually yourself, and are wondering for that reason?) It seems to me that if this is a close friend or if you have personal reason where it actually concerns your own health, then you might simply ask as directly as possible, saying something like "You are my friend, etc., and here are the reasons I feel like I need to ask you about this very personal topic..." If you don't have that personal and trusting relationship, I think you need to not be asking and respect that other person's privacy in a matter that is deeply personal.
If you are wondering because you have interest in this person romantically, I would say the best approach is to be honest about your own interest first, and take the risk in divulging your own interests and sexual orientation, rather than asking someone else to tell you what he/she is about first so that you have comfort in saying what you want to say to this person. It's hard to advise you, not knowing why it is that you think you need to understand this person's sexual orientation. I wish you well, and trust you will know how to approach it best based on what you know about the situation. Good luck to you.
Before asking, you might first be sure you have the other person's interest at heart, rather than just a voyeuristic curiosity about the situation. Do you have personal reasons to wonder (like you have been intimate with this person sexually yourself, and are wondering for that reason?) It seems to me that if this is a close friend or if you have personal reason where it actually concerns your own health, then you might simply ask as directly as possible, saying something like "You are my friend, etc., and here are the reasons I feel like I need to ask you about this very personal topic..." If you don't have that personal and trusting relationship, I think you need to not be asking and respect that other person's privacy in a matter that is deeply personal.
If you are wondering because you have interest in this person romantically, I would say the best approach is to be honest about your own interest first, and take the risk in divulging your own interests and sexual orientation, rather than asking someone else to tell you what he/she is about first so that you have comfort in saying what you want to say to this person. It's hard to advise you, not knowing why it is that you think you need to understand this person's sexual orientation. I wish you well, and trust you will know how to approach it best based on what you know about the situation. Good luck to you.