How do you know if you have bad/good karma?

Is it possible to know if you have bad or good karma? A lot of things have happened to me in life that is very negative. I believe partially it is my own fault because the way I am and how others perceive me, but a lot of it I don't think I deserve. I often contemplate what did I do to be on the receiving end of all my negative karma. As a result, I think negative and become very introverted and isolated. Question: Are all the bad things that happen to me are negative karma or is it "just me"? How can I change my thoughts to be more positive despite a tremendous amount of negativity in my life?