There have been a couple of threads concerning other peoples use of our maps / art on the web. If you work is copyrighted you of course want to stop people from using it (at least without permit) but even with open source totally free for use maps you probably want to know if anyone is using your maps anyway and what they are using it for. Probably would like to be credited for your work as well.
Now some people are really polite and ask you if they can use your map and tells you where it will be used and for what purpose. Unfortunatly not everybody are like that. If someone just prints your map for use in their own campaign there really isn't much you can do (and personally I don't care abut that - although it would be nice to know that ones maps are being appreciated), but if someone post your maps online you might have a little chance to find out other than just surfing on thousands of webpages looking

TIN EYE is a reverse image search engine, where you can upload a picture and they then search the web for copies - its still in beta and of course lots of stuff can't be found - if it hasn't been indexed yet or is behind "closed walls" where search engines can't go ... but otherwise you might just find something you look for ... it even finds altered pictures
http://www.tineye.com/

good luck searching