Check your poster for a NSS (National Screen Service) number. For much of the 20th century the NSS had almost total control of movie poster printing and distribution in the USA and in order to control this distribution the NSS started to stamp all their posters with a standard code. For reference, this code consisted of the year of exhibition and a number that indicated the order in which the movies were released that year.
Rely on other markings, such as the name of the printer, if you are purchasing a poster which has not originated in the U.S. You will generally find the name of the printer in the small-print at the bottom-right of a poster if it is an original.
Cross reference your poster with an established database of movie posters. By spotting differences between your poster and a respected database of posters you could successfully spot a fake. A site such as Emovieposter.com is a terrific example of such a resource.
Ensure the seller is trustworthy. This could be the simplest -- yet most effective -- piece of advice. Check the feedback a user has garnered while selling similar items if you are purchasing from an auction site or an online market place, or if you are buying in person ensure the seller is running a reputable business. A dodgy deal will more than likely see you purchase a dodgy poster.