Try to explain the concept to others.
If they understand it also, then you may be sure you have a good grasp of what is going on. If they ask a natural question, and you cannot answer, then it may help you to rethink about the concept in a new way. They can also provide a different view on it, so as to enrich your understanding.
"Ce qui se concoit bien s'enonce clairement et les mots pour le dire viennent aisement" (Boileau)
[whatever is well conceived is clearly said... and the words to say it flow with ease.]
