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?
I'm going to add a pragmatic answer, because while I think many of the other answers touched on this problem correctly, I don't find some of the explanations completely clear.

In the most basic terms 'karma' is the idea that every action you make has a reaction. If you do something, something else will happen in response. There is nothing supernatural about it, there is nothing about past lives involved, it's just a matter of what happens in response to your behaviour.

And so some of the fundamental tenets of buddhism are aimed at controlling your behaviour. This is important because it helps to control the responses to your behaviour. In other words, if you live a good life and do good things, take responsibility for yourself, treat people well, good things will tend to, but not always, happen to you. Conversely, if you do bad things and don't take responsibility for yourself, then bad things will happen to you.

So as far as karma is concerned this is pretty much all you need to know: that what I put out to the world, I get back in return.