It's pretty straight forward to most people how to guestimate when an image was taken. And many examples out there showing the way things used to be are postcards. One of the first things I do is turn the card over and read the message. What is the postmark? That gives you a definite timeline of when it was actually sent. But is that everything there is to go by? No, and frankly the postage cancel is just part of the puzzle.
Here's an example to look at. The Erie Avenue Viaduct in Lorain, Ohio. On the back is a message and a visible cancellation stamp.
It might be hard to read the third number, but this postmark is Sep 8 1920 at 5:00 p.m. from Lorain, Ohio. So in the image we see there are automobiles on the bridge of the styling that would point to the 20s. We also see boats on the canal or river.
One funny thing I notice, isn't that ship pretty big for that canal or river? A Google search tells me that a swing bridge was installed at the Black River on Erie Avenue in the 1870s. But the scale of these ships look a little off to me. So I go searching and here is an earlier example.
This is a photo of the same bridge. It's an earlier timeframe -- circa 1907. The horse drawn wagon with man walking beside is exactly the same as the 1920 image. This photo was taken by Willis Leiter who established his photo studio in Lorain in 1901. He produced postcards from 1905 through 1915.
My image was added to and colorized by presumably Hamm Studios in Toledo. All the houses in the background where added--the original image lacks those details. Bridge traffic was added, the automobile and the people walking with the exception of those people seen along the edge of the bridge in the image above. The tug and ship was added, I'm not certain that swing bridge would have accommodated that large a boat. The 1920s image is an actual image of the area from 1907.
Photo studios that produced postcards added elements to those images to either update them or enhance them. Those cars and ships where similar to clipart that we are familiar with. I'm not sure how they did it, but it was added in the photo reproduction process.
Today this area looks vastly different (image below). The original bridge was replaced with a bascule (draw) bridge in 1988.