Get a good ruler, with markings down to 1/16 inch. Put the zero mark at the center of a pin. Note where the pin nearest the 12 inch mark is. Once it hits 1/16 inch more than 12 inches, replace. I suppose you could put the zero mark at the end of a pin too, if that is easeir, as long as you measure the corresponding spot on the other end.

Google this, if you still have questions about how to measure. I have over 2k miles on my chains, and they are only half way to the 1/16 mark.