Lapizuli - 5:06 pm on Aug 6, 2010 (gmt 0)
I believe some people use Copyscape.
I used to use Google Alerts. That ended up being more work for me than it was worth, though. I ended end up investigating every suspicious incident of copying, and most of them weren't bothering about.
The other "trick" is to search for an exact sentence from your content, in quotes (not the title, though).
Another trick some folks use is to plant a sentence like "as my sweet Aunt Addie May used to say," naturally in the text of each page, and do regular searches for that.
Nowadays I just check out nearby competitors and keep a loose eye out, and maybe check if I'm wondering why something's falling down a bit.