These 3 Asians try to offer some tips:

And even they couldn't tell

https://www.youtube.com/watch?v=qW1a3UOAIaw

Is to check the serial number stamped on their right butt cheek. OR, you could strap on an alias and just ask them. When you're balls-deep does it really matter what their native language is?

spend a lot of time around and with them, along with the Thai, Vietnamese and the the Philipinas. It's a great and fun way to learn the differences.  

Hobby answer: I usually cannot tell them apart. But I usually don't care as long as I get to fuck them.

... they told a joke about how to tell the difference between an Japanese woman and a Chinese woman.  It's much better if you tell it rather than read it:

Korean and Japanese are short, but the k gals are huskier and have big bones and thick thighs, while the j gals have slender features.  
Chinese are taller with large bones and angular faces.  

koreans are as a race taller and slendar and amgular in the face then japanese and chinese.

you are a little bit biased on this one.  I wonder why?  Lol

take a looks at videos and pictures of each major  city....seoul,tokyo,bejing.. And you tell me whcih race has the tall angular look among general populace..

Sure ive seen pants bursting boner material chinese and japanese (the J Pornstar Aoi Manami...God her body and sex skills!  Her future BF or Hubby will probably die of a exhausation having to have sex with her every hour)

So not ALL Japanese are dwarves, and not ALL Chinese are Hobbits, and not ALL Koreans are elves?

Nope.  Im talking in the race as a whole. Not specific person.

By Angular do you mean that squared face that many Koreans tend to have?

so no i mean square as in chinese aka hobbits.

a square by definition has four right (90 degree) angles.  

It is very easy if you have been to Japan, Korea and China (Hong Kong) and Taiwan to tell the ladies apart and where they are from. Just a joke to say they all look the same.

Chinese typically speak Chinese (usually Mandarin, sometimes Cantonese or Hokkien)
Japanese speak Japanese.
Koreans speak Korean. They may also speak Japanese if they have spent time in Japan (Koreans are the most numerous ethnic group there), but not many native nihon-jin Japanese speak Korean.