When all else fails on a GRE sentence equivalence question—when you've employed the tactics of using key words, when you've eliminated as many choices as possible, when you've made sure to eliminate those words that make the flow of the sentence go awry -- what can you do, then? Simple. Listen to the sentence; perhaps even read it out loud. With most languages, people have the innate ability to detect when a sentence or parts of a sentence are grammatically or stylistically incorrect.
aWhat exactly do we mean by this? Well, throughout your (hopefully) many years of reading, being exposed to, and listening to intelligent communications, you've acquired what most people call a good "ear" for detecting when a sentence of parts of a sentence are good, and when they are bad. To be more concrete, ask yourself these questions to determine if, and which parts of, a sentence or passage, are correct:
If the flow of the sentence is choppy or if something does not sound right, then chances are that something is not right. Consider the two following sentences; can you spot which one of the two is correct and which one is not?
The boisterous laughter of the swelling crowd deafened our senses.
The trepidity laughter of the budding crowd deafened our senses.
If you do not know the meanings of the words boisterous, trepidity, swelling, or budding, then first consider each sentence, one word at a time. "Boisterous laughter," sounds okay, as so does "swelling crowd". The fragment "trepidity laughter", however, sounds odd, and in fact, trepidity, which means fear or distress, does not fit the meaning of the sentence at all, both in meaning and in grammar. The key fact to note here is that in addition to having to know your vocabulary and understanding the context of the passage, you'll sometimes need to rely on your intuition to arrive at the right answer, and it's only after you practice and hone your so-called inner-ear, that you will be able to zip through the text completion questions.