Hello, I'm a native Korean and this is a very interesting question!
Koreans, Japanese and Chinese may look the same to non-Asian people; however, we Asians can distinguish ourselves in most cases.
If there were 10 Koreans, 10 Japanese and 10 Chinese people in the same room randomly, I can match their nationalities pretty well even without hearing their languages.(Roughly 80%) This is possible by observing their appearance, facial expressions, outfits and makeup.
Let me tell you the brief general impressions on each ethnic groups.
Most Koreans are descendants of Northern Asians.
Koreans generally have smaller eyes without double eyelid with higher cheekbones and noses.
Koreans are generally taller than Southern Chinese, Japanese and Vietnamese. (Actually, South Koreans are the tallest in Asia!)
A typical Korean-looking male can be found here:
Japanese generally have larger eyes and many of them have snaggle-teeth. Japanese women tend to put on thik makeup so that they look palely white.
Their behavior is different as well. They are more reserved and avoid physical contacts.
A typical Japanese-looking female can be found here:
China is a huge country and it is not a single-ethnic(ethnically homogenous) country as they are in Korea and Japan. Northern Chinese and Southern Chinese look quite different and they speaque different languages which are not mutally intelligible.
Southern Chinese are definitely shorter than Northern Chinese and they have larger eyes and darker skin colors. Southern Chinese look more like Southeast Asians.
Mainland Chinese people don't dress well and less fashionable than Taiwanese, Koreans or Japanese.
Vietnamese people are noticeably different from Koreans, Japanese and Northern Chinese because they are Southeast Asians. They are generally shorter and darker. They've got larger eyes too.
PS. It is not easy to tell one's nationality(ethnic origin) by seeing a single side-shot picture but I guess he's either Korean or Chinese.