Historians (but not only historians) have a variety of ways of identifying the dates of objects and photographs that lack any verifiable notations. One method is to compare an object of unknown date with a similar object of a known, verifiable date. Another way is to examine closely the details of the object or photograph.
For example, the members of the Travelers Rest Historical Society http://www.travelersresthistoricalsociety.org/ recently enjoyed a presentation by Kyle Campbell of Preservation South, who is leading the effort to restore the Society's Spring Park Inn https://www.travelersresthistoricalsociety.org/spring-park-inn-project to its late nineteenth-century appearance, in which he discussed several examples of these very date-identification methods that he used in determining what the Inn should look like, what paints to use, what furnishings were appropriate, etc.
His recounting of the process began with a cursory comparison of the building's exterior with photos of similar area structures of known dates of construction. Included points of comparison were the twin chimneys that bookended the house, the railings on the front porch, the number of panes in the windows, the appearance of the glass in those windows, and even the lightning rods atop the ridgeline of the house.
The comparison process began in detail under the house with examination of the support pillars and beams; continued within the house with analyses of the various layers of paint, examination of window sashes and panes and layers of wallpaper and carpets, and comparison of numerous pieces of furniture. And it proceeded even into the attic, where roof rafters made of mere tall, straight pine saplings were discovered, and onto the roof, where wooden shingles were found and from which their size, shape, and spacing were used to duplicate new shingles.
But even if you aren't a historian or historical restorationist, you, too, occasionally use the same methods, perhaps without even realizing you are using historical processes. If you're looking through old, undated family photos, for example, how do you determine the approximate dates on which each was taken? You look at the details in the photo, and they give clues that help you narrow the date.
You look at the automobiles in the background. You examine the clothing the subjects are wearing. You notice their hair styles. And you use many such seemingly insignificant details to determine, if not the exact, at least a good approximation of the date on which the photo was taken.
As a case in point, consider the accompanying photo of my father with his mother, Omega, and Aunt Madge. Without even knowing my father's age or date of birth, you might be able to tell roughly when the photo was taken by looking at the clues. The ladies' clothes might be a dead giveaway: the shoe styles, the length of the dresses, those "skullcaps," the short hair styles, etc. This photo apparently was taken during the Roaring Twenties or early Depression. I can guess by his size in the photo, that my father was probably about two years old at the time, give or take a few months. Since I know he was born in 1927, I can narrow the date even more to about 1929 or 1930.
But the photo doesn't have to be old and historic for you to use this method. I have often determined when more recent photos were taken by looking at the cars and recalling when we bought or sold them or noting the presence of young children and when they were born. "That's the 1965 F-85 Olds, so the photo had to be taken sometime between 1977 and 1979, when we sold it." Or, "That's our old Chevette, so the photo had to be taken before Tisha was born."
It's fun to use these "historians' methods" to identify the dates of things. You've no doubt used them yourself. If you don't think you have used them, dig out the old photo albums, and you'll soon find yourself using them even without thinking about it! (And you might even discover a few memories you can share in writing!)