The best way that I have found to check if a site has duplicate content or not is by taking a random phrase found somewhere in the content and selecting it and searching for it in quotes on Google or whatever search engine you're interested in.
For example: if you have you could check if this tip is duplicate content on Google by selecting the phrase "found somewhere in the content and selecting it and searching" and then searching for that exact phrase on Google.
If you find no results then it is probably not copied from anywhere - at least that the search engines know of
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Another service that works decently well is http://www.copyscape.com/