A common tool in quality assurance is the Go/No-Go gauge. If the tool fits, it's a "Go"; and vice-versa. This is a Go/No-Go gauge for your chain. Insert the two ends of the tool into your chain and push down gently. If you can get the tool to drop over the chain on the 0.7%-1.2% side, the chain is worn and should be replaced.

Note: This has been updated and now features a black-oxide finish for better legibility.