How can you tell apart a Korean, Chinese, Japanese, Vietnamese person,etc...apart from each other?

WITHOUT asking them bluntly?
=)

And just for fun,...what type of asian do you think my friend is? haha:
http://i198.photobucket.com/albums/aa116...

thanks!
Update: P.S Sorry for the way I worded my question. lol

I wasn't thinking straight. xD
Update 2: hahaha... since everyone wants to know what type of asian my friend is, I'll tell you.
=)

My friend is......................CHINESE.
xD
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