If you have the jpg have a look at it in a text editor as a kick off. There is sometimes some tell tale information at the beginning of the file that will give an idea as to what program was used.

If you want some more information about fakes you might also want to google "The Fake Detective" where there is some interesting information. It is an adult site as it was originally set up to investigate the veracity of supposed naked pictures of celebrities that were usually photoshopped fakes. There are some good pointers on there if you can live with the subject matter that might be helpful.