JPEGsnoop will compare 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!).
Simply open an image in JPEGsnoop and scroll down to the section titled *** Searching Compression Signatures ***. This option can be enabled/disabled with the Signature Search item in the Options menu.
The assessment line indicates one of four possible outcomes:
Class 1 - Image is processed/edited Class 2 - Image has high probability of being processed/edited Class 3 - Image has a high probability of being original Class 4 - Uncertain if processed or original