1. Exactly this. Just eyesight & time.
2. Not to mention that some insects even have a bit of contrast between the lenses so it's easier to understand they are compounded.
3. And additionally due to individual lenses compounded eyes arent smooth - by reflecting light at different angles you can make the "bumps" obvious.
4. Also if there is like a water droplet on grasshoppers eyes you can clearly see it's surface structure. Just like you can see individual pixels on your (high dpi phone?) screen the same way.

Tho I bet they didn't study this ones eyes:

It's called a fairy wasp (wiki/Megaphragma_mymaripenne) and it's only the third smallest insect known.