[detokaal]

You have to know what you don't know. In other words, you need a sense of the gaps that exist in your knowledge and then fill those gaps. This is a side effect of...

http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect

... and a key part of continuing education in any field. The best way for students of any age to grasp this is to take practice tests. Look at what you missed, and go fill in that gap. A similar method it to look at a completed project or paper and wait for the aha moment, "I didn't know you could do that." Then you go find the answers.