In general, Japanese girls have very small palms (hands), their ears stick out to the side moreso than Koreans or Chinese, they have teeth that look unkept as if they don't regularly go to the dentist - not dirty, but not aligned properly in the mouth; plus they are the shortest and have the largest calf muscles of all the asian girl. Also, if there is a big titted Asian, most likely to be Japanese as they have the genes in their blood-line that produces the highest percentage of big-titted girls. And with the Japanese, because they don't like to mix with outsiders, these things tend to be very common amongst the entire population. Well, at least 1 or 2 traits per girl.
Korean girls, without surgery, are just plain unattractive. So, if you see a very unattractive Asian, she is probably Korean. They are most likely to have slanted eyes as in the old 'slant eyes' racial slur that many caucasion people once used 30 - 40 years ago about Asians. Of course, most Koreans that you will meet in this sex industry have had extensive surgery, so they look much better. I can't stress enough how ugly Korean girls are without surgery. Regular Japanese girls are ugly too, but ver short with the small palms, so you likely won't mix the two up even thought they are both ugly.
If you see a naturlaly beautiful, nearly perfect in every way type Asian lady, most likely Chinese. Chinese are the top of the food chain when it comes to Asians. There is no substitute for Chinese women.