If your postcard has a stampbox, it may have words or marks denoting what paper was used.
Real photo postcards are actual photographs that have been developed onto photographic paper.
They are generally the same size as standard vintage postcards (3-1/2" x 5-1/2").
The scan at the top of this page is a real photo postcard; click on it to view a larger image.
The easiest way to tell if a postcard is a real photo or not is to look at it with a magnifying glass.
Though most of us today are familiar with the concept of photo grain, this is mostly because we have experienced very large prints made from small 35mm negatives.
But even here the effect is more of a softening of detail than a observable texture.
Early real photo postcards are small by their very nature and since most were contact printed, not enlarged, there is no visible texture.
Postcards that are NOT real photos are made up of many small dots.
If you look at a real photo postcard, the image is solid (no dots).