Explain it to someone who will interrupt
Understanding feels identical to having heard something clearly explained. It is not.
Do this
Try to explain the concept out loud without notes, ideally to a person who will ask why. When you hit a point where you can only repeat the phrasing you read, that is the boundary of what you know. Go back to exactly there.
Why it works
Self-explanation and explaining to others both improve learning in controlled studies, largely by exposing gaps that fluent reading conceals. The illusion of explanatory depth is well demonstrated: people confidently rate their understanding of everyday mechanisms highly, then fail to explain them.
Where it fails
The benefit comes from generating the explanation, not from performing it - reciting a memorised explanation does nothing. Evidence for the specific 'teach it to learn it' claim is weaker than the general self-explanation literature.
How you’ll know
Count the places where you say 'it just does'. That number is your real gap list.