METADATA and Altered images

[Terry Felter]

I have from time to time run onto hoaxers that claim that "Photoshop" or "Ducky" in an images Metadata area proves that the image has been altered. Now from what I've read those data indicate only that the software that converted the image to digital was "Photoshop" or other. What I'm asking how would it be possible to ascertain whether or not a file was altered either using this information or in conjunction with other data in the file header?