I'm not much into "sizing people up" consciously - you'll generally get a feel for people, but I don't have any sort of training to pick up advanced little nuances that give insight into someone's character.
While most people stare blankly forwards at their shoes as they move through life, a few select people seem to have eyes that have a lot of awareness.
This "light of awareness" is actually an amazing predictor of curiosity, intelligence, and a person just being overall interesting.
What is it, exactly?
Hard to say.
It's a certain curiosity, a registering of the environment, something... partially it might be metaphorical, it might not be just in the eyes exactly. But it's definitely in the mix of eyes, posture, facial expression, and movement - and if you can pick up who has it, you'll meet a lot more interesting people.
Sometimes you walk into a room, and only one person out of many has the light of awareness. They're probably the most interesting person in the room.
Occasionally - rarely by accident - you'll be in a room where everyone has the light of awareness going on. This is incredible.
It's something worth looking for. When you're in a space with a lot of people and everyone is harried and frenzied and unengaged, and you're calm and taking in the environment and spot someone else who is the same - very likely you could click, and very likely they'll have interesting things to say and enrich each other's lives.
I don't really size people up consciously. But the light of awareness? Yeah, look for that one.