We all read people to an extent. Behavioural signs will show to a degree their emotional state, their status, their attention and interest in you/the conversation.
Combine it with how they use language and you get a good idea of how they think and how self aware they are. You can see if people monologue at you or if they are interested in turn taking. You can get a feel for how quickly people can grasp information, how relaxed/restless they are, how internal they are, how nice they are, how insecure they are, how aggressive they are etc.
I suppose I also look also for how real a person is. For example in a work setting some people are much more prone to wear masks and fake emotions and some people don't do that. I do try to factor in how much games playing some people do/don't do.
I remember being amused to learn that psychology folks call a real smile a “Duchenne” smile, and that the tell, to them, is that a Duchenne smile forms creases around the corners of the eyes.
To hear Ekman, father of the Facial Action Coding System, tell it:
That is true. But also covered in my last point about people wearing masks to an extent. If you have a limited amount of time with someone you can be fooled but a mask "can" slip if you really get to know someone. But yeah it's not fool proof and people can definitely be fooled. But having an understanding of the power dynamic between yourself and the other person can help.
But yeah some people can hide negative emotions (e.g. sadness) very well.
I suppose I also look also for how real a person is. For example in a work setting some people are much more prone to wear masks and fake emotions and some people don't do that. I do try to factor in how much games playing some people do/don't do.