If a newly discovered photo of a famous person has pencil writing on the back, that’s a mark against it being real. Why? Ink is easy to date, and the age of lead is almost impossible to measure. So forgers often use pencils.
You should check out author Joe Nickell – he got started as a forensics investigator and then moved on to investigate historical document forgeries and real-life X-files (always with a skeptical mind of course).
pt. I. Documents — Investigating documents — Diary of Jack the ripper — Novel by an American slave — Lincoln’s lost Gettysburg address — An outlaw’s scribblings — Out of the archives — pt. II. Photographs — Photo sleuthing — A second photo of Emily Dickinson — Likenesses of Lincoln — Assassin or look-alike — From the album — pt. III. Other artifacts — Authenticating artworks and other artifacts — Lost icon found — Jefferson Davis’s musket — Debris from the Titanic — Off the shelf.
Highly recommended for the historian’s thought process. Clues reside in the strangest places: a turn of phrase, hairstyle, kind of stamp, anachronisms, as well as technical analysis of ink, paper, handwriting styles and more.