I know a lot about people I don't know.
There are actually several different layers that people have at play at any given point. It's that top semi-superficial layer that I can tap into in knowing about a person I don't know.
All I really know, is how a person is presenting themselves in that current moment.
Anybody can do this, but I do it like second nature. I don't even think about the fact that I'm doing it. For example; If somebody is presenting themselves in a timid way, I can sense the level of timidity, the level of legitimacy it carries to a certain degree, and the amount of an emotional affect it is putting onto that person among other things. I don't even think about it, I just know it.
It's actually more useful when people are interacting with each other rather than picking out a single person. I see the interplay and how the dynamic is going. There are always curveballs, though, because people present themselves in different ways for so many different reasons, and without knowing them personally I have no idea what those reasons are or what they might do next. As I get to know a person, I can better gauge these things.
It doesn't flow as well when I'm directly interacting with somebody because my cognitive resources are being used on my own stuff, but I still pick up a lot without thinking about it in the slightest and I go away from the interaction with a lot of processing about the person once I can put all my brain back into analyzing the interaction.
A lot of people do not even think about this stuff or in this way at all, which is an important thing I have learned. This is one thing that makes me/infjs oddballs.