How to detect if a photo has been tampered with

JPEGsnoop is a free utility that identifies edited digital photos by comparing "the compression characteristics of the photo against an internal database of thousands of camera 'signatures' to locate a match. If a match is found, the matching digital camera or editor is shown. If the signature matches a photo editor (such as Photoshop), then there is a good chance that the photo has been edited (i.e., not original!)."