How to Master any Subject!
You can't really learn anything well until you interact with the subject matter, and experience it with others.
To know you've mastered your subject, try this technique: Explain it to an 8-12 year-old child as a well-formed story that they find interesting, and are able to easily visualize.
Now ask: Do they understand all that I'm saying?
You'll know you've succeeded in mastering your subject when you hear their questions clearly reflecting the topic at hand.