There are stereotypical Japanese and Koreans looks, just like your stereotypical Swede vs. your stereotypical Spaniard. With Koreans, it might be higher cheekbones, smaller eyes, and taller noses. With Japanese it might be a flatter face etc. But it’s just a stereotypical look, any single Chinese or Japanese could look Korean, etc.
But there are some rules of thumb, Someone who is tall, has very light skin will almost never be southern Chinese. Some one who is very dark and short will almost never be Korean.
Other hints: Any guy who wears a beard or a ring beard is almost certainly Japanese, it’s really rare to see Koreans keep facial hair. And if you see an Asian guy with a lot of facial hair (shaved or not), he’s almost certain to be Japanese, I think that might be because of blending in of Ainu blood.
In practice, many times you can cheat and tell by clothes and how they dress up. In the US, you can often tell immigrants because they still wear what they bring, especially people from China, because the increased costs of living means it’ll be a while before they can buy a new wardrobe.
One thing about Asian kids born in the US is that they can look different from Asian kids born in their native lands. This is not some unfounded ignorant comment, but it is cause for a lot of speculation amongst first generation immigrants here. The elderly have a theory that it has to do with the water(?).
Another example of that is that Chinese born in Korea often look Korean. (There was a huge exodus of Chinese to Korea after the Chinese Communists took over).
And like some of the above posters said, the Chinese are not one big group of people. There are 7 different “races” of Chinese, of which 90% are Han which is everybody in regular China, and then there are 6 others, all of which are trying to separate and form their own countries! Obviously they don’t think of themselves as Chinese. These others include Tibetans, people from Siangkiang, etc.
Even in the Han “race” there is a big difference between many of them. Northern people look very different from Southern people etc. I once met a woman from north of the Black Dragon River (which was the subject of a National Geo magazine last year). Even though she said she was Chinese and spoke Chinese she looked completely different than any other Chinese I had ever seen, she was very very white, had a round face, and very small eyes.
And then you might meet the Hakka people who have a history like the Jews in Europe. They originally lived in the middle or north of China but kept driven south. However, they managed to keep their customs separate from the other people they lived with and didn’t blend in. Even their name means Guest People.
And also, the Chinese people we know of today only fairly recently moved down to the south of China. They used to live up towards the northern part. As they came south, they drove the indiginous people into IndoChina, or killed or blended in with them. But pockets of the indiginous people remained. I think they have protected status now. One interesting group is called the Miao, who have their own customs and again look very different. I have only ever met one Miao person and did so at a banquet here. Her husband said, yes, no one could ever tell she was Chinese.
So, to answer the OP, how do Asians tell each other apart?
- Clothes, what they wear is a big clue.
- Stereotypical looks is a big clue, not always accurate, but added with 1) increases accuracy.
- Once they speak, that clinches it. There is another poster here who knows Koreans, he may confirm this. For some reason Koreans cannot say any word starting with Z. It has to do with their language, but after you listen to one, you can always tell a Korean person because they all say it the same way.