The answer is that you can't. This doesn't imply, however, that the question isn't important. In fact, it is vital to your future success because it will keep you learning and unlearning.
One way to remind you of "what you don't know" is to create an anti-library — a list of books you haven't read but which likely contain some information, knowledge or wisdom that would be helpful to you.
I really like the idea of anti-library because it can serve as a tangible reminder of "what we don't know." To this end, enjoy this picture to the right and let it serve as a visual reminder of all that you — and I — don't know.