Just google "x verifier" and you'll find quite a few. The ones on jsfiddle are the easiest to see the source code of, although personally I'm a fan of dicesites.com which has a verifier there. It doesn't appear to be opensource, but it doesn't particularly matter if you trust it to be independent from the site you're verifying. You can also just save the page and run it offline if you're worried. The site's code is also pretty readable if you're familiar with jquery  (after throwing it through a js formatter...)