instead of trying to pick out the errors "cold" (something that's very difficult for anyone who is not a professional writer or editor), you should look for HINTS in the answer choices. in other words, you should learn to interpret HINTS in the answer choices.
here are a few examples: * if you see a verb switching between singular and plural, then find its subject -- there's probably an issue with S-V agreement. * if you see ANY pronoun at all, then look for pronoun errors. * if you see a parallel signal (and, or, not ... but ..., etc.), examine the resulting parallelism. * if you see modifiers moved around (but written in the same way), figure out which version places the modifiers closer to the things they're actually meant to modify. * if you see modifiers written differently in different choices, think about the rules for the use of those modifiers. etc.