Ask what most of the details explain together. A main idea covers the whole passage without adding unsupported claims. A single example is usually a supporting detail.
Worked example
If a paragraph describes labeled shelves, a tool sign-out sheet, and sorted supplies, a fitting main idea is 'The workshop uses systems to keep materials organized.'
Repair Before Replacing
At a neighborhood repair table, volunteers help people mend useful objects. One volunteer replaces a loose button on a coat. Another fixes the handle of a basket. Visitors watch and try a step themselves. A board lists simple checks, such as looking for loose screws. Volunteers explain that some damaged objects need a trained specialist. The table helps visitors learn when a small repair can keep an object in use.