Article in English: You can try to smell your own breath, but you can only ever approximate what someone else smells. The best way to know for sure is to swallow your pride and ask, "Be honest. Does my breath smell bad?"  Choose a person that you trust--someone that won't go around telling people, and someone that will be honest with you about your breath. Ask a close friend who you know won't judge you. Avoid asking a crush or a romantic partner, as serious bad breath might be a turn-off. Avoid asking strangers, unless you're feeling particularly bold. It may seem embarrassing, at first, but you may find it a huge relief to get a trusted opinion on the matter. It's better to hear it from a close friend than from, say, someone you'd like to kiss. Don't just breathe into someone's face and say, "How does my breath smell?" Bring up the topic delicately, and always ask before demonstrating. If you spend a lot of time in close contact with the person, they may have already noticed that your breath smells bad; they may have just been too polite to bring it up.  Say, "I'm worried that my breath might smell bad, but I can't really tell. This is embarrassing, but have you noticed anything?" Say, "This might sound strange, but does my breath smell bad? I'm taking Jenny out to a movie tonight, and I'd rather deal with it now than wait for her to notice."

Summary in Chinese:
找一个你信得过的朋友或者家人，询问他们是否觉得你有口臭。 巧妙地提出请求。