To check filament diameter, I created a small Arduino sketch to measure this with an Infidel (YouMagine – InFiDEL - Inline Filament Diameter Estimator (lowcost) by Thomas Sanladerer – YouMagine 🔧), an Arduino and LCD-display. The Arduino does some statistics to make the display more useful. Alas the display is a bit small to show more accuracy, so it might be an idea to have a plugin and send data over the serial line. It could be a nice addition to a print job report too.
The data could be used to compensate extruder feed, but buying quality filament is probably a simpler solution.