"If you want to master something, teach it." - Richard Feynman

"To teach is to learn twice over." - Joseph Joubert

If you want to test whether or not you know a subject, or if you want to reinforce what you've learned, then teach it. Teaching a subject or skills will test your knowledge and understanding. Your knowledge will need to be comprehensive and refined in order to explain it to others. Even as a student, you should fine opportunities to tutor classmates or younger students because it'll help you become a stronger learner.