Assuming you have done your homework and are prepared for the test: if you see something and you have no idea how to do it, I promise you that is most probably a very simply concept that you are overlooking - take a breath and think again. It's either stated in the passage or it's a very simple logic/concept that you can deduce from the question stem. Now you might be thinking that I'm crazy and the MCAT is supposed to be hard, but from my point of view, MCAT absolutely loves to take a very simple concept, worded it such that you feel like it's the hardest thing in the world and makes you feel like it's probably something you didn't study.
1. Look at the question again, what is the question EXACTLY asking you?
2. Look at the answer choices and extract as much information as possible.
3. Refer back to the passage, could it be possibly associated with any part of the passage?
4. Take a deep breath, think again, what could it POSSIBLY be?
5. Remember my advice above - it is most probably something easy that you are overlooking and NOT something you forgot to study
6. Still have no idea? Eliminate the obviously wrong answer choices, guess, and move on.
What I personally like to do in these cases is that I immediately mark these questions and move on. I usually have 20-25 minutes at the end with 3-5 questions like these that I focus on first, then review the other unsure ones.