I can’t really see each topic having a comprehension test. Simplest would be to have a minimum “read time” based more on “in the browser” than any guarantee that any was actually read.
The only thing that I can think of that I ever needed to do that was even remotely related was a type of CAPTCHA
“The fourth word of the second sentence in the third paragraph is ____”
Maybe a “the last longest word in the OP is _____” wouldn’t be too severe? (substantiated?)