How can I find out if my photos have been stolen?

I was reading this question a href="http://answers.yahoo.com/question/index;_ylt=AvUGSW8Sb4t0FSvHVUF44AKt5HNG;_ylv=3?qid=20110701102036AAHtiLr"http://answers.yahoo.com/question/index;…/a and started wondering if any of my photos have been stolen. The person who asked the question says he deleted his Flickr account because some of his photos were stolen. How would he know his photos were stolen? How can I go about finding my photos where they shouldn't be?