Pedro is like that. Totally spot-on. It's a gift and a curse.
In fact, he can tell a lot of things about people at a glance. He can also tell at a glance if a little kid has autism. He can look at a group of kids and tell you which one is the "leader" and why the other kids follow her. He can tell within five minutes if someone would be a good programmer without asking him a single technical question. He can look at a couple of college kids in a restaurant and tell if they're friends or lovers. He can even say with confidence, "she thinks they are just friends but he wants it to be something more." Of course there's no way to test this kind of thing in a random setting...but he's so reliable with his other observations that I can only assume he's right about these kinds of things. He give compelling arguments.
"Look, she's ordered a burger and fries instead of a salad, so she's obviously not trying to impress him. She's doing most of the talking but he's acting really interested, which would probably not be the case if they were a couple that had been together for a while. Also, she's wearing jeans and a nice blouse, but he's got on khaki ##### and a dress shirt, and -- see the crease in his #####? -- he ironed them. She didn't do it for him, or she would have ironed her own clothes, but as you can see her blouse is wrinkled. Finally, she's talking to the space to the right of his head, which indicates that she is using her left brain to recount things that have actually happened, while he is looking directly at her face, giving her his full attention. So my guess is that they're friends, they've known each other a while, but he wants to date her and she has no idea."
I think it comes from being incredibly observant and also really well-versed in reading and understanding how people think and live.
He is like Adrian Monk, but with people instead of things. And without the debilitating OCD.