It used to be easier to tell where someone was from. People immigrated and stayed in the new place, or traveled for brief periods and then went home. But more and more often, people live for extended periods in other places, and even move again to someplace else. Multiracial people are common, and clothing has moved toward a more universal style. Often, the best way to tell where people are from is by listening to them speak, though some really fluent people manage to appear to be native speakers of multiple languages.
In the 1980s, before this sort of migration was very common, I met a young woman who personifies this for me. She had long, red hair and freckles. I believe her father was German and her mother, American. She learned German and English at home, and her American English was excellent. But her native tongue was Portuguese; she grew up in Brazil.
There’s a new category for this type of people: third culture kids. They are typically born into a family from Country A, but do most of their growing up in Country B.
The video below is 40 minutes long, and I find it fascinating. It explains why we ought to understand other cultures and languages: skills such as flexibility, problem-solving, and cross-cultural communication are 21st-century skills. Click Presentation (on the left). The part about third culture kids is at 14:30–30:00.