I've been looking for such a tool as well and http://www.plagspotter.com/ was a great solution. It can show you what sites on the Web have published content from your site on their pages.
Try this http://www.copyscape.com/ or this http://www.plagium.com/
Here is a system that a customer of mine has used called turnitin. This is designed for students but might be of some assistance...?
There are a couple of ways.
1) grab a whole paragraph of text from the webpage and the do a google search with it. It's simple and grude but not 100%
2) Otherwise go here: http://www.copyscape.com/
Hi Anil, I think this is what you're looking for:
(its a site that finds duplicate content and it works quite good for my sites it has find quite some content out there).
Then to protect you, you might look at:
https://www.google.com/search?q=content+checker+tools&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a Copyscape is the one I've used