There are many "free" online paper checkers that can help you avoid or detect when your work might contain plagiarized phrases or content. As with spell checker features in word processing programs, the authors' own intent needs to be considered.
Click the link above in this box to go to the Purdue OWL webpage for more detailed information on paper checker programs.
This is another great source from Purdue OWL on how to avoid plagiarism in your work. The link above takes you a page with a multitude of exercises to help the writer better understand what constitutes plagiarism.