One of the growing problems on the Web is the plagiarism of content, usually through content scrapers or similar.
If you want to check whether your content has been stolen and is being used by another website, we recommend using either Copyscape, fairshare, Plagium, or for a particular article, a free service such as article checker.
Copyscape offers both free and paid services. With search credits for individual page searches starting at only $0.05 (minimum buy 100=$5), it is an inexpensive and efficient way to check for anyone plagiarisng your content. And Copyscape is the gold standard for copy checkers.
Fairshare is a newer service but does offer an interesting beta recompense scheme for those whose work has been re-used elsewhere.
Plagium requires you to paste in the text rather than the specific URL (as Copyscape does), but seems to give more detailed results because it does not check any of the navigational elements on the page.
If you suspect that your work is being re-used, or have evidence that it is being, there is a practical guide to dealing with Plagiarism by Copyscape which includes details of how to file a Digital Millennium Copyright Act (DMCA) infringement notice with the search engines.