Try to put the material you have studied into your own words, as if YOU were explaining it to someone else, without reading it. If you can do that, you have understood the material. If you can't, you need to read over it again.
Make flash cards and study right before you go to bed. Like RIGHT before! Basically, while you sleep, your mind processes through information. So if you study right before then, you could remember it a lot easier.. Also don't stress too much about it, there has been times when I know the content, but I do terrible because I'm nervous. Look over your notes/ flash cards in the morning also.