Ask what most of the passage explains. A main idea covers the central pattern; a detail supports one part of it. Avoid a summary that focuses on only one small example.
Worked example
A passage about labels, shelves and a return box may mainly explain organizing a shared collection, not just the color of one label.
Original fiction: The Shared Supply Shelf
The art club changed how it stored supplies. Labels showed where each kind of material belonged. A return box held items that needed sorting. A picture chart helped new members find tools. At the next meeting, members found the tape and brushes without asking three different people. The changes did not add new supplies; they made existing ones easier to find.