Is it a gelatin/silver print? or perhaps it’s an albumen print? Tintype? Is it some sort of mechanical print and not an original photo at all? How do you tell?
Graphics Atlas is a great place to begin your research.
“Graphics Atlas is a sophisticated resource that presents a unique, object-based approach for the identification and characterization of prints and photographs.”
From the folks at the Image Permanence Institute at RIT and the George Eastman House.