Thankfully, this is a pretty easy one to answer, at least as far as the SAT is concerned. There is an iron-clad rule of thumb that you can take into test day. If you can rule out at least one answer choice, guess. If you can't, don't! Simple, huh? If you're the curious sort, and want to know WHY this is the rule, read on.
The reason behind this is how the SAT is scored. In a nutshell, you earn one point toward your raw score for every question you get right. If you opt to leave a question blank, you'll earn no points. If you answer incorrectly, you'll be penalized a quarter of a point. So, lots of wrong answers will just hurt you in the long run. Hence, random guessing is a really bad idea. If you're clueless, just leave it blank.
But, if you can rule out at least one answer choice, you should guess. It works this way statistically. On the SAT, you've got five possible answer choices. If you eliminate one of those five as wrong, you're left with four possibilities. Even if you guess randomly from the remaining four, you're likely to guess the right answer one out of those four times. Now do the math. If you earn one full point for that one time you get the right answer, but lose only a quarter point for the three times you guess wrong, you still end up a quarter point ahead. Your odds are even better if you can rule out two or three wrong answers.
So go into test day feeling confident you can guess with the best of them!