Antique platters often have the manufacturer's or artist's name imprinted on the bottom along with the country of origin. Older platters may have more obscure markings that are almost always on the bottom as well. Painted platters may have the artist's name in the one of the lower corners. Value is dependent on age, rarity, materials and pattern.
The pattern of an antique platter can be a very good indication of its origin and creator. An antique dealer or auction house can offer an appraisal and more information. If there are no markings at all, owners may have a hard time determining the identity of the item.
EBay.com and AboutAntiques.com have listings of antique platters that can offer some insight into the identify and value of a piece. It can take some time to determine the origin of some platters. Online collector forums and Facebook pages can offer some insight when users post photos of platter is question.
Counterfeiters have introduced fakes of some popular pieces over the years, so for pieces that appear to be of great value, owners should get the professional opinion of an expert or auction house.
Sites like AntiquePlatters.com provide examples of an array of platters for sale, which can be useful for price comparison.