there is a website where you can post your essay
Yes, the one Laura is probably trying to remember is turnitin.com.
But I have heard of dustball, too.
As far as I am concerned, the best way to check something is to google some selections of text and see what comes up. Just highlight a sentence from the essay and copy/paste it into the search box.
:-)
I'm going to move this thread to "student talk"