Usually, I can't tell if a person is bi-racial or multi-racial just by appearance. Also, some people may not see themselves multi-ethnic. The Hui people in China, for example, may be genetically similar to the Han people in China, but ethnicity is not based on genetics. It is based on culture and way of life. What makes the Hui people different from the Han people is that they use Arabic as a liturgical language and behave like Muslims. Being Han and being Hui is mutually exclusive. Also, as far as the Chinese government is concerned, a Chinese national is either one ethnicity or the other, and can't be both at the same time. This video is about "ethnic Russians" in China. From a Western POV, they would be considered bi-racial. From a Chinese POV, they would be considered mono-racial/mono-ethnic. They are classified as 俄罗斯民族 (Russian ethnic group), and because they are part of 中华民族 (Han people + all recognized ethnic minorities), they are considered "Chinese". Another example is this girl (https://www.youtube.com/watch?v=if-n2AdVpUI). The video uploader adds the title: "Native Chinese white girl(native white race in China)", contrary to the evidence. First of all, the news reporter and the girl never mention that she is "white". She speaks her own ethnic language, and that she is an ethnic minority in China, specifically Tatar. She does mention that when she moves to Chongqing to study, she receives attention from the other students and locals, because her outer appearance looks like a foreigner. The Tatar people have varied appearances. A Westerner would probably look at the appearance and, if one person "looks Caucasian" and the other person "looks Asian", then the child would be bi-racial. However, I don't think that works for the Tatar people, because the child would still be regarded as mono-racial: Tatar. The parents' physical appearances make no difference. Then, there is this girl (https://www.youtube.com/user/DrVanharanta). She considers herself "half Finnish and half Hongkonese". I think, if you live in the West or have a very Western mindset because of one European parent, then you are more likely to identify yourself based on physical appearance. So, in the West, people do identify themselves as "bi-racial" or "multi-racial", and part of that self-identification has to do with physical appearance. Basically, China is "multi-ethnic" as a nation, but individually, every single "Chinese" person is mono-ethnic/mono-racial regardless of physical appearance, unless the Chinese person marries a foreigner. I remember in a YouTube video, a guy identifies himself as half-Chinese and half-Japanese, seeing himself as mixed-race. That would make sense, because Japanese people are not Chinese, hence foreigner. Westerners, on the other hand, would probably see him as "monoracially Asian".
So, no, I can't tell if a person is bi-racial or multi-racial just by appearance.
However, I will say this. Writing fiction is about the narrator's POV, not about political correctness. The only time when political correctness matters is when your audience is the social justice warrior type of person. A SJW reader is the type of reader that wants to see people of different races and sexualities mentioned explicitly. In their minds, they think they are being "inclusive", even though they are just assuming a Western viewpoint. Some people just don't see themselves as "bi-racial"; some people do see themselves as "bi-racial". In terms of sexuality and gender, Westerners will emphasize one's personal feelings and individualism. How one identifies oneself on the gender spectrum is more important than how society sees the person. A person may in fact identify as male, simply because society (relatives and friends) identifies him as male, and he looks like one on the outside.
If you know someone who is bi-racial or bi-sexual, then you can just write about your own personal experience with that person. The narrator is you. The bi-racial or bi-sexual person may be a friend or some kind of acquaintance. There you go, that's how you describe a bi-racial/bi-sexual person.