^ It just takes research. You need to know the exact sizes of the original posters, because reprints are commonly different. One-Sheets from the mid-80s and before measured 27x41, and most reprints are in the frame-standard size 27x40. Of course, if a person is intentionally forging posters to pass-off as authentic, then they will custom-print to size. There was a huge
scandal with a major forgery ring that got their hands on the original printing plates and equipment to do just that:http://www.learnaboutmovieposters.com/N ... Report.asp
If the posters are older, you have to get a feel for recognizing how the paper should look after aging as long as it has. In the U.S., most posters from the 1980s and earlier were printed by National Screen Services alone, not by other companies or the studios themselves (as is common now). There are specific sets of markings that NSS would put on a poster, and if they're not there or aren't correct, it's probably forged. Of course, you need to know what the correct markings are to check that. You can find a lot of good information for beginning collectors about U.S. posters here:http://www.learnaboutmovieposters.com/n ... /intro.asphttp://www.filmfan.com/
When you start buying posters from other countries, you'll need to research their practices in the same way. I don't have a link for that, but its the kind of thing that you pick up with time. Don't be afraid to ask dealers for more information, the honest ones will be happy to oblige.
Also, Bruce Hershenson's website, eMoviePoster (http://www.emovieposter.com/
), is the absolute best source in the world for poster collectors. He auctions the posters himself, the individual sellers do not, and that means that Bruce authenticates every single item. The site NEVER sells reprints. I bought my Gigantis the Fire Monster insert-poster from him, and many other non-Toho items. Toho posters do turn up regularly though, I usually see several auctioned per month. The auctions are handled a bit differently than eBay, more like a traditional, real-world auction. There's also a selection of fixed-price items to buy outright, if you don't want to bid.
Despite the effort it occasionally requires, authentic movie poster collecting is a way to connect with films in a much closer way than buying licensed merchandise and modern, for-consumer prints. It links us back to the heyday of a motion picture, and the theaters and patrons that they touched. Also, by buying a vintage poster and caring for it properly, collectors can actually do a service to history, getting these artifacts out of basements and warehouses, and preserving them for years to come.