Squint test

Designers borrowed the squint test from print. Scale the thumbnail down to roughly the size it appears in a feed, or half close your eyes at it, and see what survives: one face and two or three words, usually. Anything that only works at full size is decoration.

The test catches a specific failure, which is designing on a large monitor where thin text, a busy background and small detail all look fine. It measures legibility and nothing else. A thumbnail can pass the squint test cleanly and still promise a viewer nothing worth clicking, so it is the second check rather than the first.