EPC stands for electroplated copper. Sheffield EPC refers to Old Sheffield Plate, a form of plate silver popular in the mid-1700s up to the 1840s. Though the term EPC refers to electroplating, the technique used to create Sheffield pieces was mainly mechanical and chemical. Early Sheffield plate is notoriously difficult to date because there was no legal requirement to hallmark an item until 1894, according to Bryan Douglas Silver.
Flip your silver item over and look around the surface for a hallmark. If you discover the stamped initials "EPBM" or "EPNS," then it is not copper Sheffield plate. Instead it is likely silver-plated nickel from the 19th century or later.
Search for a symbol or initials on the metal. Old Sheffield Plate is usually identified using certain names and dates imprinted on the metal. Matthew Boulton, James Dixon and Sons, Law Thomas are three examples of famous Old Sheffield Plate manufacturers. Marks include two sun-shapes for Boulton. Many other makers have their names in small shapes or shields.
Check the maker name against the date listed in a hallmark guide such as "Miller's Silver and Plate Antiques Checklist" or "Bradbury's Book of Hallmarks."
Note the decoration. Items in a gadroon rope-twist or shell border are usually Regency style, so likely date to the 19th century. Plain angular designs are often from a later period, such as the art deco style of the early 20th century. Neoclassical styles such as small bead borders or column candlesticks might indicate a piece from the early 1800s.
Speak to a specialist. Many auction houses or dealers will value your silver items for free, often on the expectation that you might sell it through them at a later date.
Pieces marked "old Sheffield silver" are in fact reproductions.