Basically, from what I can tell, it comes down to three things:
Know the game rules
Know the Floor Rules
Know the Compendium
Of course, you have access to these materials during the test, but it's rather difficult to go looking through them during a timed exam (which is what the Professor Test really is). You don't really need judging experience (I passed it twice without ever having judged an event), but it will help. Talking to Professors, TOs, and PTOs in your area will also help you in some of the "more subjective" questions. Good luck!