Photo mechanical (Photo Lithography, offset printing, half tones)
During the very late 19th century and all through the 20th and 21st century -- up until quite recently-- almost all printing was done by photo mechanical means. During this era increasingly complex and high speed presses and film based cameras were developed to print books, newspapers, magazines and everything else in bigger, faster and cheaper press runs.
On the graphic side the half tone dominated the reproduction of images. Half tones are an ingenious way of simulating graduations of tone by means of dots. The dots are the giveaway that it’s printed by photo offset.
When you see the dots in an image you know right away the image is almost certainly 20th century and it’s made by a photo mechanical method.
Caveat
There are lots of quality small press run books printed by photo offset and there is some exceptionally high quality printing of photographs and art work done by this method using half tones. It doesn’t mean they’re not nice books, and it doesn’t mean they are not rare, scarce and valuable books, prints or maps. But if you see the dots it does mean that they may well be modern reproductions of older works.
Screen Printing
You’re not going to see much screen printing of books, but you may see some screen printing of posters and ephemera. Most especially you’ll see sought after screen printing during the mid-20th century when a lot of the posters, t-shirts and other items associated with popular culture were made by this process.
Some of these things have become expensive and hard-to-find. If you ripped a vivid poster for a Grateful Dead concert off a telephone pole in San Francisco or waved a printed banner during a Viet Nam era anti-war rally, chances are good it was a screen print. Screen printing in the mid-20th century was often short run. . It was the technique of choice for the underground because it was cheap, easy and most often done in the garage after hours. More recent screen printing is an entirely different animal. It can be done in vast quantities and on practically any surface.
