From the excellent Farnam Street blog:
The famous Nobel winning physicist Richard Feynman understood the difference between knowing something and knowing the name of something and it’s one of the most important reasons for his success. In fact, he created a formula for learning that ensured he understood something better than everyone else.
The formula is simple. First you write down what you know about a subject as if you are teaching it to a child. You'll inevitably find gaps in your knowledge, at which point you'll have to go back to the source material until you can explain the whole subject.
To really internalize the knowledge, you then rewrite your notes into a naturally flowing story. Your ability to do that is an indicator of your mastery of the subject.