I can tell the difference...I think.
Korean girls don't pluck their eye brows, they mostly have their hair black, and they have a different fashion style, but there are a few who likes Japanese fashion.
Taiwan/HK follows Japanese styles.
Hk girls have bigger and flatter noses and more Tom Boys from HK.
Chinese don't have any fashion style, at least the ones I've seen.
Lastly...I take my guess and then listen carefully to which language they speak.
Its hard to tell them apart if they have no fashion style.
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Its hard to tell them apart if they are just pictures from online..b/c of photoshop.
Photoshop is beauty tool of an Asian girl.
Korean:

Taiwanese:

Japanese:
