Yes, at least sometimes.

Yes, usually. Korean and Japanese women can sometimes be difficult to tell because they tend to wear a lot of make up. Contrary to popular stereotypes, I can't really tell by the eyes, as much as from facial features over all. Again, women tend to be difficult to differentiate due to make up and the growing trend of plastic surgery, "double eyelid" glue, etc. Style (as someone mentioned) can be a big clue. I won't go into detail, but generally this is the biggest difference, especially for females.

Alan, some basics about foreign cultures help)

Yes..but sometimes no. I can see the difference between the Chinese,Korean and Japanese. This is because: 1- Almost everyday I see Chinese at my place. (At school,shopping mall,public etc) 2- I often watch korean shows/dramas and the men are the most significant if compared with Chinese and Japanese. The reason is........... [i won't tell it here. ] 3- hmm..i can tell that Japanese is kind of different~ Their fashions are different too. BUT...most of them(Chinese,Korean and Japanese) have slanting eyes. :D that's the common~

I can (as a Chinese).