You can put in gear and roll bike backwards to tighten upper run of chain. Then measure distance between 24 pins. Brand new-chain measures to exactly 15". Wear limit is 15-3/32". If it measures more than that between 24 pins, time to replace.
Might want to implement regular chain cleaning & lubing routine. I lube every other tank, clean once a month. I like to use wax-based lube after cleaning with kerosene. Original chain is not stretched straight, some links are kinked, indicating lack of smooth rotation around pins. Increases wear significantly.
JT, long-time maker of sprockets, has their own X-ring chain now. I'm going to try it next time around.