https://www.sheldonbrown.com/brandt/chain-care.html
Don't bother with a chain checker unless it is the Pedro's or the new Park. All of the others are not accurate and have you replace a chain before it's worn out.
https://www.parktool.com/product/chain-checker-cc-4.
https://pedros.com/products/tools/ca...ecker-plus-ii/