This may sound nuts, but I think people's faces reflect the language they speak. Perhaps because of my Pacific Rim base, I find this particularly obvious in the faces of Asian women. A huge number of the people here in Vancouver are of Chinese extraction, resident for periods anywhere between four generations and a few weeks. Being a normally male sort of person, I'm given to looking closely at women's faces. And quite often, when I look, I can tell instantly "she speaks generic North American English" or "she's a recent immigrant and has a heavy accent."
I can't tell what the clues are that are giving this away. Intuitively, it would seem more likely to be something in the clothing, hair-style or makeup that would do it, but it feels like it's deep in the facial expression.
Here's a story: one time I was in a crowded sushi joint in Tokyo near Yurakuchō JR, the kind with the little boats that go around and you take what you want. I was by myself reading a magazine and suddenly amid the roar and bustle a woman's voice rang out in a purely American laugh. I hadn't noticed any other gaijin, but I scanned the room and sure enough, two-thirds of the way around were two Anglophone women, their genes as Asian as anyone else in the room (except me), but with New World faces.
Another: right now I am sitting in the departure lounge in YVR waiting for a plane to Chicago, and there's a party of four across the lounge: a young white fellow, his smoothly-English-speaking Asian girlfriend or wife, and two other Chinese-looking women, one older (her mother?) and another younger. These second two ladies have little or no English, and the wife/girlfriend is interpreting. I can't hear what they're saying from here, but I know I'm right.