The KMC chain checker I used in the video: amzn.to/3y3J7eh
In this video, I run through the process of measuring drivetrain wear on a badly worn drivetrain, specifically the cassette. In the case of this bike, it was pretty obvious everything waist need of replacement, but sometimes , it can be tough to tell where the line between "replace" and "it's got 1000 more miles left" is at. It really starts with chain wear. Lots of folks say to replace your cassette every time you replace the chain, but from what I've found, you can get 3-4 chains from each cassette if the chains are replaced regularly.
I like using the KMC chain checker for it's accuracy and definitive number instead of an arbitrary percentage number that most chain checkers use.
Regardless, I like to perform a stress test on every bike I replace the chain on. You can tell many times visually if a cassette is bad, an sometimes not, but the true test is the stress test as demonstrated in the video: Ride slowly with the brakes on and lots of pressure on the pedals through all the gears.