How to find someone's nationality?

Have a DNA test done

Ask them. I do, and I'll tell you why. As a writer, I have a deep interest in accents. When I hear one I can't identify, I simply ask them their nationality. Never had anyone get mad at me yet.

DNA won't tell you where they are from.  For example, you are probably American, but you could have English or German ancestry, or Polish, or Italian, or Spanish, or Asian, or African, or Native American.  The surname may give you a clue on the father's side, but a German name could be from Austria, Czechoslovakia, Switzerland or Argentina.  If you want to know, you have to ask.  You may get a very involved story that the grandparents were from Czechoslovakia, but they got out before Hitler invaded and went to Romania, and escaped through Italy and Switzerland.  Or the person may tell you it's none of your business.

"You may get a very involved story that the grandparents were from Czechoslovakia, but they got out before Hitler invaded and went to Romania, and escaped through Italy and Switzerland. "

Ah, that's the fun of it for me. It's the best way to learn history, from the aspect of how events impacted various people's every day lives.

PGroot, To explain my answer regarding DNA:    I've been watching on PBS, the finding your roots program on Sunday evenings 8pm...that's what they do to find out how much Native American; African; Asian; European a person has in them.  So, it does work in those cases.

You can find a lot of information on  Ancestry.com
www.ancestry.com/FreeTrial
Search for your ancestors & build a family tree free on Ancestry.com.