And even they couldn't tell

Well, at least one of them was over 50%

end results

It's freaking hard

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.

Non hobby answer:
1. Facial features alone
Some typical features will give you likelihood but never guarantee. For example, Koreans and some northeastern Chinese look quite alike. Also, it's easy to tell Japanese/Korean from Southeast Asians. Not true for some Chinese.

2. Name alone
Japanese names are clearly distinguishable.
Korean and Chinese names are usually distinguishable if the full name is written. A bit harder if only last name is known because some can be identical.
Kim/Park: Korean
Anything with an "x": Mainland China (think Chinese President Xi)
Li: Mainland China (think Jet Li)
Lee: Korean or non-mainland China (think Bruce Lee)
Li and Lee are the same name (李) spelled differently. Koreans don't actually pronounce it "Lee" but since this character has a well-known English translation, they stick to it.

3. English Accent
Obviously doesn't apply to American-born or those who immigrated as a child.
I haven't had much interaction with Japanese, so not sure about them. But once a Chinese or Korean person speaks, I can tell immediately. If you're good at either language, you'll notice they speak English very differently.

... 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:

Q:  What's the difference between a Japanese woman and a Chinese woman?
A:  The Japanese woman says "Nipponese" (nip on these, pointing to her tits) and the Chinese woman says "chu manchu" (chew, man, chew, pointing to her pussy)

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.  

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.

There, THAT was easy...