AllLookSame
Being Asian doesn't give you an insight, but working and living with them does. I have lived in Japan and taught Japanese to many Thai, Korean, Chinese (mainland, HK, and Taiwanese) and most of the time I can tell. The first clues are always fashion, but even just by facial features and skintones I can usually guess where someone is from.
That has nothing to do with profiling and stereotypes. It's generalizations based on experience. The Korean male chinline and eye-shape is often (not always) distinct from the Japanese. Skintones vary among the groups as well.
I have included a link to a site which tests this very question.
I have a lot of UK blood, and was often mistaken for British or Irish when I lived in Japan, mostly by UK residents. They saw something familiar in my features, I guess.