Well, I'm American (European by descent) and I can still tell sort of a difference between the Japanese, Chinese, and Koreans. It's about looking for different physical characteristics...I can't really think of how to describe it, I guess you could say that Chinese tend have more "rounded" facial features while the Japanese' are more "sharp" as far as the curvature of the eyes, nose structure, etc. It's something you just have to develop an eye for. So the Japanese would be able to tell based on looks that you are originate from somewhere in mainland Asia and not Japan.