One suggestion that has been already made on the list is to check for the number of total shutter releases on the camera. That's an internal number that on most DSLRs doesn't get reset even if you reset the frame # counter, and is stored in the EXIF field, usually in the makernote. I don't know of an EXIF viewer that supports it for Pentax JPGs but I am sure there is one.
Good light!
fra
