David,
Suppose you wanted cover "X,W,and Z". Could you conjure up 5 or 10 bits of information they ought to know? If so, you could begin the class by giving a non graded exam, and as a review, discuss each answer with the class as an exercise. This might take 10-15 minutes, but you'd be fairly well informed where that particular cohort of students level of understanding is.
Barry Westling
