Three big purposes
Most passages do one of three things: explain something (inform), argue something (persuade), or tell a story (entertain or describe a feeling). Identify which one before looking at the choices.
Read the verb
The choices usually start with a verb: 'to compare', 'to argue', 'to describe', 'to demonstrate'. Pick the verb that matches what the passage actually does, not just what it talks about.
Detail vs. purpose
Wrong answers describe what one paragraph does, or what the topic is. Right answers describe what the WHOLE passage is trying to accomplish.
Tip: revisit this strategy after every 10 missed questions on this topic.