I use http://www.copyscape.com/ and http://www.google.com/alerts/ Theres also a free javascript out there that adds random letters all over your content if someone copies and pastes it, so its basically worthless to be copied as they will get an "a" "b" etc in random locations.
This is only marginally better than worthless though. If they cut and paste the content from the browser window then it might work. If they take it from the source, as I suspect all self-respecting pond-life content thieves would, then it will not work.
You can always send them a nice cease and desist letter (just google it, you will find bunch of "templates")
To determine if your material was copied, go to copyscape and enter your urls. You can also enter your text for comparison at http://www.webmasterlabor.com/tools/checker
have a look at this article timeforblogging.com/2006/12/10/how-to-protect-your-websites-copyright-when-someone-steals-your-content/
There is no way to know if somebody has taken content from your site. I can go into your site, highlight a section I like, hit "copy," then paste it into my site. Your best bet is to ask your visitors if they have seen copies of your work. Most Webmasters find out from their visitors. Even though I am talking about my entite site, look above my navagation box on the bottom of the page as an example. http://www.thelist.fm/Dating-Singles-Women.shtml
Give me your url, leo, and I'll go in and copy some of your text and copy it to my site. Let's see what happens, shall we? ~
Sure you can do that - your objection was, 'there is no way to know if somebody has taken content...' Well, I would certainly find out, where you place that text. It is an open question, what I would do then, of course!-)
All said and done.. we still cannot find automatically if the content has been stolen. The pocess is manual and tedious, specially if it involves multiple sites and loads of content.