My personal benchmark for this has always been: Can you communicate it to a lay person? Then you really understand it.
Also, remember in the real world, we keep reference material around all the time.
For me, I can maybe understand a concept if explained in simple words, but when there are like 7 different letters involved with word symbols and less than or equal to signs everywhere with different intervals with epsilon and delta, etc..... It's hard for me to grasp the concept without getting lost in all that mess. And then when trying to remember it later, all I do is end up memorizing the sequence of letters put together making up an equation instead of actually understanding how the equation works.
So how can I study to make sure I can explain it to someone rather than using super complicated terminology