Answer:
Well, there are many ways to do it so. Basing for how they act, for instance. Focusing on someone's action, you can tell how do they express their emotions. Seeing a child stomp their foots when their favors wasn't granted, you can tell that they're upset. Second thing is their expression (This depends because some of us know how to mask emotions out of our face). Seeing someone bit their lower lip can either tell you they're frustrated or angry.
