Early deck designs were frequently copies of previous deck designs. In Rouen, France, they'd make cards to sell to the British, who would copy them, make them on their own and sell them to the Americans (and probably the Canadians as well!), who would in turn copy them and make them on their own to sell to whomever wanted them. It's interesting because much like a child's game of "Telephone", the design often changes just a little from one copy to the next, and the accumulated changes can make for a much bigger difference from the original generation. Sometimes this was intentional, the artist's way of putting his own imprimatur on the work, while other times it was simply a series of errors in copying.
Consider the King of Hearts - when first created in the Rouen style, he didn't have a sword behind his head (not through it, like most people believe when they call him the Suicide King). Instead, it was an axe carried over his shoulder. You'd never in a million years guess that was the case based on common, present-day designs.
To a limited extent, you can try working out the printer and the age of the deck from those changes, but it would be more like guesswork than an exact science, especially when factoring in designers' proclivity to copy designs from other designers.
If it was a later deck, and you knew the printer, you'd have a better chance of using courts to determine age. USPC courts are pretty distinctive and have gradually changed over the years - there was a really good article on this in Clear the Decks a few issues ago.
