[drcy]

The back of photopaper before 1968 should be fibery. If it's smooth and plasticy on back like a 1999 Kodak snapshot, it's after. The fronts can be (and usually will be) glossy. In 1968 Kodak started making resin-coated paper, which means the paper back and front is very smooth and plasticy, though not all post 1968 photopaper is resid coated.

The back of the photopaper generally gets darker the older it is. 1930s is usually darker than 1950s-60s than 1990s. It's not dramatic, but if you directly compare to a 1990s or 2000s photo (say a family snapshot) to a 1950s-60s photo, the 1990s or 200s photo will be much brighter/whiter. This is part because the paper naturally tones with age and part because in recent decades they started bleaching photo paper which made it bright white.

I did a comparison with a whole mass of photos 1910s-modern, and while it wasn't foolproof, the tonal changes were consistent. Even a 1988 versus a 2005 photo was usually different tonally. For testing tone, you should do direct comparison with other photos. While a 1910s or 20s photo will be obviously well toned at first glance, a lot of more moder photos don't look toned or off-white until you directly compare them (literally one overlapping the other) with a modern photo. This is also just one test, and you should relay on one test-- but I've found it very helpful.

The chemicals that make photoper fluoresce brightly under UV were introduced circa 1955. Most photopaper after will flourese under blacklight, but not all.

When you get more modern photos, it is harder to date the paper-- which is nice when it has stamping or tags.