It's not uncommon that popular sites are often plagiarized word for word. It's probably happened to you, as much as it has to us. You might find tools like CopyScape offer some useful automated tracking but it's limited in scope. If you're looking for more, consider Tynt.com's Tracer (Beta).
What does Tracer tell you? The Dashboard gives you a tag cloud of the most copied words, a list of the most copied images and pages, as well as high-level stats for page views, copies, etc. over different time spans.