While that's a difficult question depending on your meaning, you can get some info at http://copyscape.com. I paid for some premium credits personally, allows you to automatically check every page of your website.
This is a very complicated subject, the best thing you can do is to document very carefully the process of creation, time stamps and so on, and then to use those to challenge the people that cloned your content.
Mostly in the context of college, but useful stuff anyway:
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[ 3.1 ms ] story [ 19.9 ms ] threadEdt: clickable link.
You are going to have to prove 'provenance'.
http://en.wikipedia.org/wiki/Provenance
This is a very complicated subject, the best thing you can do is to document very carefully the process of creation, time stamps and so on, and then to use those to challenge the people that cloned your content.
Mostly in the context of college, but useful stuff anyway:
http://www.bobbyelliott.com/Plagiarism.htm