It's just a combination -- I take in the whole package. It's not even like I want to. It's just second nature. I can just tell if something is bothering someone. I can FEEL someone's wall they put up. It's like a huge elephant in the room. And it makes me feel the way they do. Some of the cues might be: they avoid eye contact, their answers become short, there is a solomness to their face, they don't talk as much, they don't have reactions like they normally do. (Ex: maybe they are more prone to texting, smiling, or doing whatever...and they just stop.) It really depends on the person. It's easier to tell when you know them better. But it's really just a combination of cues, plus also my own experiences of how I might react when I'm upset that just cements the fact that I'm right that there is, indeed, something bothering them.