Through physical appearance, I agree there is no way you can tell Eastern Asians apart, but depending on how they "package" themselves is another story, usually from the clothes, make-up(girls), hair color/design, and even eyebrow design(guys). Strip them from all that and you'll see no noticeable difference, period.

80% of the time I can tell the difference between someone who is korean and Japanese. Its hard to tell the difference between white people because a lot of them are mixed. I don't know anyone who is just from one background.