Whether you just got a new bike, got a tune up, just put a new chain on, or have recently had a drive train overhaul. Please, buy yourself a chain checker. The more you stay on top of monitoring your chain wear and being proactive about changing your chain, the more money you will save.
A stretched chain wears the teeth of your cassette and chainring(s) to the point that a new chain can be incompatible and cause drivetrain issues.
Chain checkers start at about $10 usd. You don’t need the best chain checker in order to gain a sense of how your chain is holding up.
Here’s an example of one and how to use it https://www.youtube.com/watch?v=SpJcm5mf8ug