Don't even try; it never pays to try to second-guess the computer. There's no point in wasting time and energy wondering wehter it's feeding you harder questions or easier ones. Let the computer track of how well you're doing- you concentrate on answering questions and pacing yourself.

Should I Guess?

Yes, you must! You are not going to know the correnct answer to every question on the GRE. That's a given. But you can't just skip a question. In order to move on to the next question, you first must answer the question currently on screen, even if you haven't a clue as to what the correct answer might be. So if the question on screen has to stumped, eliminate any obviously incorrect answery choices, and then guess and don't worry whether you've guessed right or wrong. Your job is to get to the next question you can answer. Just remember to use the process of elimination to improve your guessing odds.