One technique he often deployed in round the classroom "point and test" scenarios was to ask you a question and when you answered it correctly stare at you as if you had somehow not quite got it completely right. At which point you invariably uttered "um" and became vulnerable to further quizzing.
The point here, as he would explain, was that your belief in your own knowledge should be strong enough to stand up to further scrutiny, and hence his summary:
"It's not enough to know. You have to know that you know".