<p>My friend got in trouble last year for copying a site. Another got in trouble even though he didn't plagiarize anything, and it took forever to clean up.</p>
<p>There are a number of sources on the Internet that will search articles, books, etc and compare it to essays. One such, I believe, is called turnitin.com. Some high schools use it as well.</p>
<p>You cannot protect yourself from baseless accusations. There's no way to avoid being accused of something you didn't do. All you can do is make sure that you're on the straight and narrow and then defend yourself vigorously if an erroneous accusation is made.</p>