Alicent in episode 1 sees Criston Cole take off his helmet and says "gods, he's DORNISH!" once she sees his facial features. So characters can notice someone's ancestry or where they come from simply based on their face, just like how in the real world I can tell someone is probably from the Philipines if they're tiny asians with high hairlines.

And a scene later Alicent sees a black guy in a white mop, blacker than any Summer Islander, and just calmly accepts him as a cousin of the Targaryens (the whitest family in Westeros.)

This show is dumb sludge written for low IQ phonewatchers.