I think it's a combination of instinct and observational skill. There are always a few core qualities of a person that I feel like I can pick up on by observing the eyes: cowardice (tbh, I feel like I detect this more amongst men than women), honorability, anger, fear, and confusion. I could probably list a few more but I feel as though those are the easiest to detect.
Discussion
An interesting list of emotions to pick up on, maybe it's a manly list even, like those things matter more to you.. Fear and confusion are easy for me too... I always know when someone wants something from me long before they say anything. Also, being closed to new ideas is a personality trait 1 that I pick up on often. It's where people sit on the Openness - Closed spectrum... Maybe I'm just a purveyor of bad ideas, but being unwilling or unable to discuss new concepts makes people boring. Sadly, too many people are boring or unwilling to rock the boat. Give me a good mental work out any day.