This article does a great job highlighting an instance of how it feels when someone jumps to a conclusion about your identity. I am American and have raised 2 daughters in Germany who speak perfect English and perfect German and look like ordinary Germans. When people hear them speak English, they sometimes get asked "Where are you from?" to which they respond "Munich." People often don't believe them and insist that they must be from somewhere else. My daughters have encountered this frequently and find it very offensive.

The right question would be "Where did you learn to speak English so well?" which is fine - and neutral.

I was surprised when they told me this story, but I have never experienced such a thing personally. They assured me that when you encounter such behavior again and again it starts to get old - and offensive.