Well that all depends on how much you pay attention to other people. You can catch little signs on people that will tell you exactly how they are feeling. You just have to know the person well enough to tell. I had a best friend once that knew my every mood even when I was trying to hide it from him. I asked him once how he did it and he told me that my eyes shift colors with my moods. He was the only one that I couldn't really lie to. So it is possible to do you just have to know where to look on that person.