And of course any image that contain a regularly patterned series of dots is not a photograph at all but a ink printed image.
Some halftone cards were printed on high gloss paper to resemble a photograph but their screen patterns will give them away if one is vigilant.
Today there are many real photo postcards of unknown origin and date.
When no postmark is available, the type of materials used can often aid in narrowing down the years it may have been produced in.
This too is not foolproof for many publishers had large stocks of photo papers using them for decades after they stopped being manufactured.
NOTE: There were many other photo papers manufactured in addition to those listed on this page, and even these could be made in different finishes from matte to glossy.
While today this would lead to lawsuits, copyright was uncommon and rarely enforced at the turn of the 20th century.Collotypes, which provide the finest detail of all printing methods are sometimes confused with real photo postcards.But even collotypes will exhibit a discernible grain when magnified.Most old photo papers used silver in their emulsions.As time passes this silver tends to migrate to the surface of the print creating tell-tale metallic patches.