I had a boss from Korea while working for the Japanese company, Konica. My brother came for a visit and we went to an oriental restaurant. The girl who took our names and got us seated had roundish facial features like the Japanese and somewhat flat cheeks like Koreans.
I figured she was from somewhere between Korea and Japan, so out of the blue, I asked her if she was from Hong Kong. Yes, indeedy. she was. My brother’s mouth dropped open and he asked, “How did you know that?” I just smiled.
She was just being polite to the stupid Gaijin. (She would otherwise be fired in a New York minute.) Believe me: She was actually from, e.g., Okinawa.
I find it actually rather boorish to ask a total stranger such a personal question. What if they were to smilingly answer, "I wonder about that myself sometimes. You see, I was abandoned as an infant, raised by gypsies, and then sold into prostitution at the age of 13. So I really don't know."
When I bump into an English-speaker abroad, and they comment, e.g., "It's mighty cold here!" I might respond with "So where abouts are you from?" - But to speculate openly about their genetic make-up as a conversation-breaker?!
Regards,