Then you need a device to measure sound frequency, and a convenient solution is to use a smartphone application. Gates (the belts manufacturer) provides a free app specifically for this purpose, and it works fairly well. Just launch the app, bring the microphone close to the belt, pinch it, and check the measured frequency. The value will vary depending on how you pinch the belt, but by doing it a few times a reasonable average value should emerge, like in the example below where I got a majority of "109Hz" values: