How does one tell, Will? Well, when they come into the classroom saying, "Please, Sir, can we do "Hobson's Choice" today?" it is a fair indicator. I am not saying that happened all that often; but you can generally tell when the book we are reading together is grabbing them. (Funny how I fall into the historic present here: it is all more than 25 years ago!)
A shame you had Cole's-Notes-bound teachers, Sins. I tended to go to the other extreme and go into background to the extent of having some pupils ask anxiously "Why are we talking about this? Will it help in the exams?" But I generally managed to cover the syllabus. I believe I am right that the National Curriculum would probably now preclude my wandering off to talk about the Pre-Raphaelites for a double period because of a stage direction in Pygmalion about William Morris wallpaper or such; but I never learned to regret such digressions.