Measure the Chain with a Chain Checker

A chain elongates as its pins and rollers wear, and a worn chain quietly grinds down the cassette and chainrings with it. A go/no-go gauge such as Park Tool's CC-3.2 drops into the links in seconds and settles the question without guesswork. On 5- to 10-speed drivetrains, replace the chain at or just before 0.75% wear; on 11- and 12-speed, at or just before 0.5%. A chain at the limit is a cheap fix. A chain well past it often means a new cassette too, so price that into your offer. SRAM Flattop and T-Type 12-speed chains need a different gauge.