Ok, talking about university physics. Usually what you can see in class of any certain topic is not everything there is to know (they are far more deep). My auto-test for knowing if I got it (after acing the exams, of course ;) ) is this: Can I rewrite the theory? When I am able to develop the theory myself, writing all the "intermediate steps", and knowing how to "triangulate" concepts, is really useful.. Can I do all the exercises? Can I program something that would do the work for me? If I can make a program that can solve the problem for me, I'm usually done in the topic. Having made the program, I don't even bother in remembering the "special techniques" used for solving the problem, because in the future, I can just run the program! Do you know any interesting clever way to know if you really understand something? In the way that you internalize concepts and you can use them in the future without trying so hard.