A chain measuring tool is quite cheap & will let you know when your chain has stretched to the point of needing changing
Measuring tools tend to the pessimistic, and have been known to show an unused chain as needing replacement (they measure between rollers, and they move about on the pins).
A better measure is to use a ruler to measure between pins about 12" apart. If the end pin is less than 1/16" past the inch or half inch mark, the chain is good and the cassette will be virtually unworn. If it's more than 1/8" past, it's worn enough than a new chain will probably slip under pressure. In between,you can use a new chain OK but you will have worn the cassette a bit.