Chains don’t stretch, they wear, and if you want to find out how worn yours is, you could use one of these BBB chain checkers.
Simply place the gauge between the links on a section of chain – if the teeth sink into the links, allowing the gauge to lie horizontal, then it’s time for a new chain. The instructions suggest the longer measure (marked .1.0) is for use on bikes with steel cassettes and the other with aluminium, but I’m inclined to use the less generous measure, meaning the chain gets replaced before it wears out the more costly drivetrain.
In practice it’s easy to use, gives an accurate at-a-glance reading of the chain’s health and works fine on both 3/32in and 1/4in track chains. The nickel plating seems a little thin on the test model but this is a very minor point. Decorative cut-outs might not be to everyone’s taste but they do allow it to be kept on a key fob – a bonus if you’re prone to losing tools.