Mastery begins with the ability to explain a concept clearly. To use the Feynman Technique, take any idea and attempt to articulate it as if teaching it to a child. The point at which the explanation breaks down is the point at which understanding is incomplete, and these are the areas that require further study. Return to the source material with precision, then test the explanation again. I think true understanding is demonstrated when you can explain complex concepts with simplicity, showing that you have fully grasped them.