Suppose I'm talking to a person who is an American citizen but obviously has some "Asian roots" (either they immigrated to the US at early age or their parents were immigrants). Is there a polite way to ask about their ethnicity (if that's the right word)? If I just ask "Where are you from?", they will definitely say they are from the States since they grew up here.
I'm neither American nor Asian, and I don't quite understand why such questions are thought of as being rude by default. The scenario I can think of does not have any negative connotation. For example, if I live or lived for a long time in say Korea or have a Korean spouse and if I suspect that someone I'm talking to (in the US) is of Korean descent, then knowing the latter may help find us some topics for discussion in which we are both interested.