Too loose a belt, it can skip. Too tight, it can stretch and affect accuracy, and create more wear on parts. The most common way to test this is to pluck the belt and use an app on your phone to measure the frequency. Then hopefully someone can tell you what frequency it should vibrate at for that particular axis on your particular printer or CNC.
I can't reach the belt to pluck it on some of my 3D printers. This thumps the belt and measures the frequency. It report the frequency and calculate the belt tension. I may build a test fixture to test and measure the common GT2 belts at different spacings and tensions to come up with my own chart/calculator.
https://gatesbeltsandapplications.blogspot.com/2017/10/calculating-tension-from-hz-some.html
I'll most likely use a Teensy.
A small supermagnet on a 3D printed compliant mechanism. It will be made to fit between the space in 2020 extrusion.
Searching for a frequency vs belt tension calculator. Saving the links here with the intention to test these against a real belt later. Primarily for the GT2 belt which is very popular in consumer 3D printers and CNC machines, with the ability to input different widths as many CNC machines use wider belts.
I will mostly likely make this specific to genuine Gates toothed belts, as knock-off belts are going to vary in weight and I doubt anyone is going to pull belts off and weigh them in grams to get the grams per meter.