In order to ascertain whether you really understood what you learned, it is a good idea to try teaching what you’ve learned to others as if you became a teacher.
We all tend to "think that we’ve understood" something. It is when we try to explain it to others and fail to properly express it in words that we realize our understanding has been incomplete.
Also, in the process of trying to explain something to others, fragments of knowledge are combined and we can check for ourselves whether our explanation is logically appropriate. This deepens our understanding. Furthermore, putting into words allows feedback from others which can complement or modify our understanding.