Teach others! There is nothing that will help you find the gaps in your knowledge faster than teaching that concept to someone who is trying to learn. You will be forced to articulate all of the thoughts in your head, and simplify them so they are digestible. As you are explaining different concepts, and attempting to answer your student’s questions, the gaps in your knowledge will be come clear.
When you are able to teach others the ins and outs of a particular topic, you will be in a good place and can move on to mastering another topic.