Dating photo stock

[steve B]

It partly depends on the photo and the lab that it was made in.

Snapshots back then often had the date stamped on the back.
8x10s usually came from a more pro type photo lab, and often won't have any markings. But the paper was the same.

For a beginner, I'd suggest going to some antique places and picking up some old photos. Pics of random people and/or places are pretty common, and usually $1 and under. The more involved photos are easier to figure out the age of. If the pic is of a family standing in front of what's probably the new family car, it was almost always taken during that year or the one prior.
Another good clue is womens fashions. Especially late 40's through maybe the late 60's those can get you within a year or two.

The paper for snapshots was usually the same as for 8x10s, so once you've got a handle on what a 40's photo paper is like, an 8x10 will be much easier.

You can also look at how clear the image is. While it varies with the source material - 35mm won't usually be quite as nice as a medium format and the large format negatives mad for very nice photos- and the skill of the photographer, an image that isn't crisp is often the product of a copied negative made by placing a negative over another piece of film stock. the thickness of the plastic usually affects the "focus" causing the second generation negative to be less crisp than the original. (Modern laser techniques are MUCH harder to spot)