There are quite a few ways to detect plagiarism when it's severely suspected.
Always require a Works Cited or References page. Discuss the importance of these in class or on your policy statement. If at all possible, teach students how to cite or at least point them in the direction of style guides. Style guides are available on Writing@CSU. You can also use the Your Sources tool in the Writing Studio.
Check the availability of sources: Have they been checked out? Do they really exist? (Burwell, et al.)
Be aware of mixed citation styles. Harris notes that these could be indication of confusion on the student's behalf, but they could also be mixed direct copying of sources. To the same end, be sure that each source cited in the body of the paper is cited in the Works Cited or References Page. Also check the URL's provided; if they don't work, it could be an indication that the paper is very dated or that the sources don't exist at all (Harris 62-3).
When teaching a large lecture, it can be impossible to monitor students' writing and research processes. One way to keep track of paper integrity is it use the Internet. Since the availability of papers to be bought from the Internet and the ease of "cut-and-paste" is something our students are familiar with, be Internet "savvy" yourself (Keir).
Harris suggests using the "Google-Plus-Four" method: "Find a four-word phrase that appears to be unique to the paper or paragraph you suspect...Next, take the phrase to Google and perform an exact phrase search by typing the phrase into the search window, and surrounding it with quotation marks." If the paper is available online, it will usually come up in the search. He also suggests Findsame at www.findsame.com, HowOriginal at www.howoriginal.com, EssayCrawler at www.essaycrawler.com, and EssayFinder at www.essayfinder.com*.
Along the same lines, Keir also suggests using online scanning mills like Turnitin (www.turnitin.com), Plagiaserve (www.plagiaserve.com), or Glatt Plagiarism Services (www.plagiarism.com/INDEX.HTM) to determine if a paper has been copied or bought.
Particularly for large lecture classes, a "plagiarism detector" program can be useful. Harris includes many of these in The Plagiarism Handbook. A few examples are Plagiarism.org (www.plagiarism.org), Integriguard (www.integriguard.com and www.paperbin.com), and Eve2 (www.canexus.com/eve/index.shtml). These programs compare uploaded student papers against databases of free term papers and web page content. To use one, a teacher must usually buy its software or buy a subscription to the service and then register a class. For each assignment a student turns in, the paper must be entered into the software or uploaded to the site and then the program will run a check of the integrity of the content and provide matches to existing websites or papers.*
*Be aware that some of these are for-profit sites and do charge for their services. Findsame, HowOriginal, and EssayCrawler are free at this time.