You find out whether you really understand something the moment you try to explain it to someone else.

Explaining is the test that recognising cannot fake. Reading a page and nodding along feels exactly like understanding, right up to the moment somebody asks you to say it in your own words and it turns out to be phrases memorised in order. The gaps only show when you have to build the idea out loud from nothing.

Before your next test, teach the hardest topic to somebody who does not take the subject. Watch for the sentence where you slow down. That sentence is what to study tonight.