Just like you wouldn't date a chick without doing some background research on her, you've got to know the history of the design of the jewelry you're buying. Designs change depending on when the piece was made or sold.
Chanel, for instance, has used many different markings since the 1960s, including both round and oval cartouche signature plates and sometimes stamping CHANEL directly into the piece. The earliest pieces of Chanel jewelry were unmarked. Most pieces from the 1930 all the way to the '50s rarely come up for sale on the secondary market. These design markings obviously change the value of the piece, with older pieces being pricier as most are often harder to find. The more you know, the less chance you'll be ripped off.