Couple of good reads:
A black light is effective in identifying many, though not all, modern paper stocks. This allows the collector and dealer to identify modern reprints and fakes of antique trading cards, posters, photographs, programs and other paper memorabilia. Many people buy a black light specifically for this purpose...
http://www.cycleback.com/blacklight/optical.html
Most reproductions are made from a dot-matrix or half tone process, which produces a lentiginous image composed of a symmetrical pattern of small dots.. If you look through a fairly powerful magnifier (e.g. 10X) and you see little dots (either black & white or color), then you have a reproduction.
http://antiqueprintsblog.blogspot.co...roduction.html