Most silver carries a stamped or laser-etched mark -- known as a “hallmark” or “assay mark” -- to show that it has a legal standard of purity, which in the United States, as in most other countries, is 925 parts per thousand. Even if a piece does not have a hallmark, it is reasonably easy to tell solid silver from silver plate, which is a process of silver applied to base metal. The two have different patterns of wear.
Check for a hallmark. On U.S. silver, this will be the number “925,” or the words “sterling” or “sterling silver.” On British silver, look for an emblem of a lion walking sideways, known as the “lion passant.”
Look for areas that seem dulled or discolored and give them a cleaning. Use piece of tissue and a dab of toothpaste. If you're at a yard sale, a thumb moistened with saliva makes a satisfactory polishing implement. On silver plate, your efforts will make little difference, because the discoloration you have noticed is the result of corrosion and wear in the plated layer. With solid silver, all of the dirt will come away, revealing a shiny surface underneath. Silver is a soft metal, and the polish oxidizes dirt to reveal a new top layer.
Inspect edges, decorative features and engraved words for crispness. Polishing removes layers of silver, so you should see signs of blurring and softening of details from repeated cleanings over time. Among white metals, you will only see this pattern of wear on solid silver. With silver plate, any degradation in the thin outer layer reveals base metal underneath, usually yellow nickel.
Gently press worn areas with your thumb. Silver metal becomes thinner in places, so you should detect a slight give. You won't find this in silver plate.