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davidcyclebackBelow are two online guides that illustrate how prints of all types are examined, identified and authenticated. There are scientific, though practical, methods to dating printing and cardstock, and determining if a newly discovered card is vintage or a moden fake.
examining prints in general (Rembrandt to Warhol)
http://www.cycleback.com/printsexamination
examining trading cards
http://www.cycleback.com/baseballcards
A brief explanation is that different printing was used in, say, 1909 than in 1970, and the printing can be identified and dated with a microscope. So an expert can take a microscope, look at a newly discovered baseball card and say, "This is vintage printing" or "This is a fake, becuase this process wasn't invented until the 1970s." Almost all fakes, including the crapola on eBay, are quickly identified as fakes because they were used with a printing process that didn't exist way back when. And almost all legit early 1900s cards can be verified as vintage because the printing used went out of existance many years ago.