I wonder…are you specifically referring to understanding math concepts?
One suggestion: Students are often skilled at following procedures–seeing a demonstration with accompanying explanation (and sometimes even answering questions about what they’ve seen/heard), and then solving problems that fit the pattern that’s been demonstrated. To check how well students actually understand a concept as a concept (rather than recalling it as something to do or restate), you could try…
a. Ask them to explain the point of the lesson in their own words. For example, why does a fraction become larger with a larger-digit numerator but smaller with a larger-digit denominator? Can they demonstrate this visually with a sketch on paper or on the board in several different ways?
b. Have them practice some problems on the board while you observe. Their hesitation at certain points in their problem-solving is often an indicator of what they “get” and what they’re unsure of.
The same approach also helps with understanding grammar.