I know they do - at least in my college where an English teacher ordered us to do a homework: Find at least 5 useful online resources preventing students from commiting academic plagiarism. Some people wrote about Turnitin, some about other anti-plagiarism organizations. One student found this site and this is how I found it too. But I doubt they visit this site often - most of them don't want to know about the issue.
If you don't cheat, they won't catch you :). My suggestion is - order a paper but rewrite it yourself, using your own words and phrases. That way you will sleep better at night just like me now ;).