Most of those images you see on the Web are usually fakes and you can't really trust them. To get rid of such confusion, a handy and useful tool called <a href="http://www.pskiller.com/" rel="nofollow">Photoshopped Image Killer</a> will tell you whether an image has been edited by some software, or it's real. As indicated by it's name, the tool has best performance for Photoshop. However other image editors are covered as well.