Try and write a supplement for it. You'll be cross-checking all kinds of things that you otherwise would not think of. Instead of just reading from page 1 to 180 or whatever, you'll be flipping through the index, trying to find that one bit of information that is important for your current encounter, spell, or character ability that you want to add in.
I have found this situation really makes me focus more, and I actually learn the rules better overall. You don't need to publish your supplement, just have it for home games. But, by the time you have finished writing it, you will have a much firmer grasp of the ruleset than you did before.