If it's somebody I know well I'm usually know how they are feeling or how they will probably feel about something. My friends ask me how I think other friends will react to things because they know I'm so good a predicting how they will feel.
If it's somebody I don't know well I often have much more trouble. I find myself asking questions in order to pin down how they feel:
So that made you mad? Oh, so that made you sad? Uh, huh so that's makes you feel embarrassed?
But you titled this thread "Empathy"...does that mean I should discuss how I empathize with people? Well, frankly I have a bit of trouble empathizing I think. I understand how a person feel might feel a certain way and, if they are a friend, I want to help them to feel happy/better but I don't feel sad with them. I've told my friends that if they need me to do something for them to make their lives better I'll do it. Hell, kill a person who's driving them crazy? Sure. But I've told them that if they need a shoulder to cry on I'm not their person. Give me something to do to help them and I'll do it but I'm not good at the whole emotional support thing.