There are a number of ways teachers can figure out if their students are plagiarizing. There are a number of websites and programs that will help you do just that. You type in a portion of your student’s paper and run it through a plagiarism checker to see if those words appear elsewhere on the Internet. If they do, your student may have plagiarized.
Check out these free online resources: http://drsaraheaton.wordpress.com/2010/09/26/are-your-students-plagiarizing-heres-how-you-find-out/