In very good at it. Chinese people are less pale and have chubbier faces and fuzzy eyebrows. Japanese people's eyes tip downwards and are often more wide, they are the naturally the most palest of the group and have really jet black thin hair. I don't know how I'd describe a Korean but you'll know one when you see one. Vietnamese people are dark but have unique bone structure compared to the Chinese, like big nostrils and strong jaws.
And if you can't place an east Asian looking person in any of these categories, they are probably Nepalese.