Real Photo Cards
Among the various pages listed here you will find many examples of real photo postcards. You'll find that the quality of the cards will vary, which is general indicative of the age of the card.
See if you can find some of the initials described in the exhibit on these cards and try to figure out approximately when they were made! If the card has a post mark, you can check yourself.
The sections are roughly divided into different sections, with the photos in each section roughly joined by some unifying category. Perusal of these cards will hopefully give you a sense of the rich history of real photo cards, as well as an eye for spotting them yourself.