How do I choose between two headings that both seem to fit a paragraph?

I keep getting stuck between two answers in IELTS Reading matching headings. One seems to match the first few sentences, but another fits what the paragraph talks about later. I know I should look for the main idea, but how do I decide which one is actually the main idea? I end up rereading the paragraph and wasting time. Does anyone have a way to narrow it down?