Sterling Silver will be stamped "Sterling" or "925" or ".925" or possibly "Ster" and that mark should be accompanied by the name or trademark of the manufacturer. This means that the piece is 92.5% silver and 7.5% alloy, usually copper. The copper is alloyed with the silver during the manufacturing process and is not any danger to the user. Silver plate is a much less closely defined product, with the depth of plating varying widely. Silver is electroplated over a base metal, usually brass, which is subject to oxidation and the formation of verdigris, a poisonous copper acetate, in the presence of acetic acid, i.e. vinegar. Silver plated ware that has lost some of its silver layer should probably not be used to eat from. At best it can cause off-flavors if used in acidic foods, at worst - you did notice that verdigris is poisonous, right? Probably not terribly dangerous in the miniscule quantities you'd consume from your salad fork, but is it worth taking the chance?