Unfortunately, the time constraints of the LSAT generally force all decisions to be made kind of quickly, so sometimes it’s not “What’s the best theoretical way to do this?” as much as “What’s the best practical way to do this, given that I have less than a minute and a half per question?” One quick and dirty method to address this issue is if you think you have the main point identified, you can test it by saying to yourself, “Why?” If it’s really the main point, the answer to “Why?” will be found in the passage. For instance, Preptest 52, Section 1, Question 1 (the first problem in the “Ten New Actual, Official LSAT Preptests” book) is a Main Point question. This may be a question that is easy to you; I’m just using it to illustrate what I mean. The idea is the same for harder questions. Let’s say you have it narrowed down to (B) and (C).
(B) is that under the ranking system, the top 10% are rewarded, and the bottom 10% are fired. Why are the top 10% rewarded and the bottom 10% fired? The passage doesn’t tell us.
Now look at (C): “The ranking system is not a fair way to determine penalties or rewards.” Why? Because good workers could receive low rankings merely because they’re in a group of good workers. And because managers give good rankings to people who share their interests. The answer to the “Why?” question for (C) is found in the passage.
Not a panacea, but a pragmatic test (similar to negation for necessary assumption questions) that can often help you zero in on the main point.