of course, with the web it's nearly impossible to know if anyone is using your photo without your consent - nature of the beast, what with the billions of sites out there. If you ever did come across one of your photos being used, you could make notification and in many cases they would just drop it without a fight - they probably figured noone would find out. But they may just ignore you or put up a fight...it's up to you at that point if it's worth pursuing.
My method has always been to put pretty small versions of my photos online - I stick to 800x600 or so. That way, someone could end up using it somewhere on a webpage without my knowledge, but they likely can't make any money off selling it or printing it and passing it off as their own. Most stock agencies and photo buyers will want much higher res than that, and an 800x600 won't print much bigger than 4"x6" without getting badly pixellated.
I've found my photos once on another site - I informed the site that was hosting, with proof that these were mine, and they were removed by the site admin. But who knows where one of your photos might be lurking out there!?
I think if you include the copyright symbol and documentation on the photo and date of copyright along with the EXIF data on the original photo, you'll have sufficient proof to defend any unauthorized use of your photos...even if you had to go all the way to court with it. You still technically have the copyright on your photos as the original photographer, but it might be harder to argue your point without having made notification that the photo was copyright protected.