Hello Flash1988,
I have extensive experience at what you are wanting to do. I don't really recommend it, though, unless you have decent control, which I guess I don't.
If you want to know when one of your friends is pissed off, or feeling any other way, watch them, take notice of their behavior, their facial expressions, their tone of voice, their body language. It's not too difficult. Even better, feel what they are feeling.
This comes with a little warning, though, even if you are able to detect something is not right with them, you can only guess at what is causing it. I have made a lot of mistakes guessing at what is disturbing someone, although I have guessed more often right than wrong, you must realize that you can't know 100% what the cause is. It could be you, or it could be some person from work that they just ran into...you have to be careful about drawing conclusions about the reason. The best way is to ask. That doesn't simplify much, because lots of people lie and don't want to admit if you are the one who upset them. You have to accept what they answer, reserve your decision on whether it's true or not and usually time will reveal whether it is true or not.
To be honest, I have been trying to effect the "well, it's their problem" approach that you want to abandon. If people share their problems with you and let you in on their deep, dark secrets and start confiding things in you that they don't tell to anyone else...the fact is, you can't do anything to help them, yet you get drawn in to their lives and become somehow expected to do something and to always be there...and when you leave their presence, your mind is left in all of their heavy problems or miseries and you feel bad because you can't help them. That's why I said you have to be able to control it well...or not get with people who only come to you to vent their miseries and also don't want to get passed them.
In a nutshell, you just need to be perceptive. Good luck
.