The easiest way is to put your site on CloudFlare and go to https://www.cloudflare.com/apps/dnschanger_detector to enable the app.
DNSChanger Detector works by attaching an alerting function to the window, and making a JSONP call to a special URL. The IP resolved for that URL by the infected servers serves a file that calls the alerting function.
CloudFlare is trying to make the Internet better for everyone.
The Federal Bureau of Investigation (FBI) has been operating DNS servers to keep infected users online, but they are going to stop soon (July 9, 2012). We want to do what we can to keep people from being kicked off the Internet when that happens.
For more background, see http://www.fbi.gov/news/stories/2011/november/malware_110911
This software is MIT licensed.