After a student has presumably learned a concept, ask him or her to become the teacher. Sit back and listen to what the student thinks you taught. You'll soon find out how well the sponge absorbed the important ideas as well as the supporting details.
In fact, this process is so revealing that some designated examiners, during the flight test, ask students to tell them (essentially to "teach" them) about some aspect of aviation. This might be how the wing works, how the pitot-static system works, and so on. The wonderful thing about this type of testing is that it can be used on the spot, with little or no preparation, and can reveal both deep and modest levels of misunderstanding and confusion.
For instance, it's one thing to know how to perform slow flight. But try asking a student to teach you the main reason you'd want to know how to fly an airplane at low speeds. Be prepared to hear many different ideas. Your student might say, "Well, we fly slow to keep from bumping into other airplanes on the downwind leg," or she might say, "We fly slow so that we can put the flaps down." Then again, a sharp student might say that that we learn slow flight so we can better understand the low-speed handling qualities of an airplane, since landings are done at relatively slow speeds close to the ground. Bravo!
While there are many ways to test a student, the I Teach You: You Teach Me method is one of the most practical--and powerful.