Posted 12 February 2009 - 02:50 PM
The most common tool I hear about is copyscape.com. You enter the URL of your page, and it searches its index for duplicates. In some cases, especially if there is a unique phrase or unusual wording in a document, you can simply enter that in Google. For instance, if I am moderating a forum I will check suspicious posts to see if they have been copied from other forums by finding unusual phrasing and entering those unusual phrases into Google, in quotes.
The only way to learn anything is to question everything.