1. You can pretend to be curious and ask during your conversation one day: By the way, after knowing each other for so long, I don't know if you have a significant other? Then she will naturally give you the answer, so asking politely won't feel abrupt, and it's very natural. Whether the other person has a partner or not, they can answer the conversation smoothly.
2. When chatting with her, if you find that her emotions are not very high, then say: I see that you are not in a good mood, or your emotions are not high. What's wrong? Did you have a quarrel with your boyfriend, or something like that? If she says yes or didn't have a quarrel, it means she has a boyfriend. If I don't have a boyfriend, I usually say, 'I don't have a boyfriend yet, why are we having trouble?'.
3. Do you have a boyfriend/girlfriend? If not, let me introduce you to one! "This way, we can clearly ask if he really has a partner.
4. If you have reached a point where you are still compatible, you can directly ask her out for a date. Before the date, you can ask, "Would it be inconvenient to ask you out?" This way, he will naturally tell you if he has a partner.
