This page lists some methods that I have used to detect plagiarism in student papers. Prevention is much better than detection. Remind them that if they can find the page, so can you!
Don't assign the same topic for more than one semester. Avoid generic topics. Insist on personal experiences that demonstrate their understanding of the course material.
What I do is have them work on a group project. Then, they are to hand in a group paper containing those points that the group agrees on. Each student is also to hand in an individual paper that discusses how they applied the course material to the group project. I make it clear that I don't want a generic paper, but one that keeps refering to the group project. If anyone tries to cheat, it becomes obvious since either it (a) is generic, or (b) doesn't match with what the other team members state in their papers. The entire group would have to conspire, which seems unlikely.
I insist that students hand in both a paper copy and an electronic copy of their papers. This makes is easier to copy & paste segments into search engines; otherwise, you will have to type stuff in yourself. Also, if you decide to submit the paper to a web site, you will need it in electronic form. I keep the electronic copies, which allows me to search previous papers looking for duplication.
If you suspect some portion of the paper isn't original work, try copying a couple of consecutive sentences containing some unique words and/or phrases, and then pasting it into a search engine, surrounded by quotes. I suggest either Alta Vista or Google, which have a significant number of pages in their database. If any pages are displayed, they contain an exact quote of the text you selected.
If you don't find an exact match, then remove the quotes and try again. This will detect situations where the student copied a segment, but made some minor wording changes. This type of search will often return a large number of pages, but only the first 10 pages or so are likely to be the source. Locate the similar sections on the found pages, and determine how closely they match with the selected text.
In my experience, students don't copy an entire web page, but rather copy paragraphs from various web pages. If you don't find any matches during your first attempt, try some other paragraphs.
One thing to look for are different, often conflicting, opinions in the same paper. In one case, a student made one statement, and then a conflicting statement several pages later. Another clue is opinionated statements with little or no justification.
In one case, I found about a half dozen pages that had the exact same text. Each page claimed it was theirs, and often had a copyright statement for the supposed author.
Another possibility is that the student purchased a paper from a web site. In this case, the page isn't on the web, but rather the student is e-mailed a copy of the paper. There are a number of sites that allow you to submit a paper, and they will tell you if they found any matches. See below for a list of some of those sites.
Center for Academic Integrity
This is a site to promote academic integrity among students.
The Ultimate Plagiarism Resource: Detecting Plagiarism & Preventing It
This is a resource about plagiarism for online schools.
Steering Clear of Plagiarism in the College Crunch
This site provides an overview of plagiarism, including useful links to other sites.
Allows you to submit a paper and get comments on possible sources, including submitted papers and web sites.
The following links were obtained from an article in the Philadelphia Inquirer Tech-News section by Joyce Kasman Valenza. They are included with her permission.
MLA: A Statement on Plagiarism http://webster.commnet.edu/mla/plagiarism.htm
Plagiarism Prevention Web Page
Internet Paper Mills (a list of the cheat sites)
Preventing, Detecting and Tracking Online Plagiarism
New Plagiarism: Seven Antidotes to Prevent Highway Robbery in an Electronic Age
Plagiarism: What it is and How to Recognize and Avoid It
Plagiarism Avoided: Taking Responsibility for Your Work
Student Plagiarism in an Online World
Writing Place Tips for Writers www.writing.nwu.edu/tips/plag.html
Writing: Plagiarism Advice for Lessons (Grades 1-12)
Online Plagiarism Screening Services Eve2.2 www.canexus.com/eve/index.shtml