As an empath it is often very hard to tell your emotions from others, especially when you are not alone. There are several ways you can assess this. First of all, before you go some place with people take a baseline of your emotions. How do you feel today? What are your own emotions? Then when you are with someone or a group of people if you feel something think to yourself these questions...Did I feel this before I came into the room or got close to this person? Is what I am feeling rational? Did my mood change rapidly? If so, why? Leave the room or step as far away from that person as possible. Wait 5 or 10 minutes and ask yourself how you feel again. Do you still feel like you did near that person? If not then they weren't your emotions. I personally have noticed if my mood changes rapidly when I am near someone it is usually not me. I generally assume any emotion I feel when I am around others isn't me and that saves a lot of questions. I know what my general baseline is now and that I am alright so when I yoyo I know it is someone else. It takes a little time but once you get used to it you will do the same but start by using the questions. They help to get you started.
Taking a baseline has been helpful for me in the past too (when I remember). It also helps to think about why you could be feeling what you are feeling. If you don't know, think about why someone you are with right now could be feeling that emotion. That really helps me to know where the emotion is coming from and how to deal with it