This is a great question, since it's likely that you'll encounter at least one GRE question in which the vocabulary stumps you.
To be honest, if you have absolutely no idea what any of the words choices mean, then your only real option is to guess. If you know what one of the words means, you may be able to eliminate at least one choice, and that'll improve your odds of guessing correctly. But even if you don't know what the word itself actually means, you might be able to use your intuition, rather than known definitions, to narrow down your options. This isn't a great technique, and should be a last resort, but let's go over your options:
1) Even though you don't know the exact definition of a word, you might be able to recognize the context in which you've seen the word before. For instance, you might think, "I've heard this word used in negative situation before," or "I've heard this word used in romantic literature before." This can help you eliminate certain choices.
I would only use these techniques if you absolutely cannot eliminate any answer choices. These are the only techniques other than guessing that will be available to you, but, as you can see, they're not very precise ones. They're better than blind guessing, though!
All that said, the more words you know, the better off you'll be. That's why it's so important to work on your vocabulary for the verbal section. For strategies on how best to do so, see this post.