Many tools are available to help locate duplicate content one of the best ways is Google itself.  Copy a string of text from a web page or word document and past it into the search bar. You will see any websites using the same copy.

Other tools are available to help with duplicate content

http://www.copyscape.com/

http://www.plagiarismchecker.com/

http://www.plagiarismdetect.com/

http://www.plagium.com/

You can’t stop all duplicate content because usually it’s beyond us, submitting a press release will often see your article being published on many websites. This doesn’t mean you will get punished just some of it will be filtered from the search engines. When writing content for a website it’s always better to start from scratch this way you can be sure to avoid all duplicate content.