I recently bought a faulty filament, where the diameter changed (there were thickenings in different places). To make sure that these were really thickenings, I printed an element consisting of 36 holes of different diameters. By threading the filament through the appropriate hole, I can see if its thickness is constant or changes in different places. The project is very simple, but I think it may be useful to someone.

This tool is designed for 1.75mm diameter filament.