I like to try and decipher all Asian faces that I see. I can separate Korean faces easily from Southeast Asian faces...from Chinese, a bit less easily, from Japanese, with more difficulty. Some faces are harder than others to pick out...it depends if they look like a "classic" example of their ethnicity to me or not. I can't explain it with words very well.
I got into this habit because I've got some family members and friends who tend to just say "Chinese" when they see an Asian face, much to my annoyance. So I began correcting them. It's good practice, although it doesn't do much to train said family members and friends
That having been said...can Asian people distinguish different ethnicities of Western faces? Tricky with Americans I think, because many of us are so mixed. (I have Irish and Hungarian in my ethnic background, but I have been told I don't look "typically Irish" at all, except for my lack of sun tolerance