To be classed as silver in Great Britain and many other countries, an item has to contain at least 925 parts silver per thousand. A cheaper alternative to silver is silver plate, where a thin layer of silver is applied using an electrochemical process to a body made from an inexpensive base metal such as nickel. However, by following the steps below you should have no problem in recognising solid silver when you see it.
Check the item for any impressed or laser-etched marks. Most pieces of solid silver will bear an "assay mark," a mark to indicate that they have met the legal standard for silver in a particular country. British silver bears an emblem of a lion striding along in profile known as the "lion passant." For more help with assay marks, see Resources.
Examine the item for any marks to indicate that it might be silver plate. The letters "E.P.N.S." - meaning "electroplated nickel silver" - and "A1" are both certain indicators of plate.
Give any dark areas a brisk rub with a cloth or tissue. Silver oxidises in the air, resulting in a soft black residue -- this is why it needs to be regularly polished. If your tissue comes away black but reveals a smooth clean surface underneath, then this is a sign of solid silver. If the layer of dirt has become stubbornly ingrained in the surface of the metal, then this is an indicator of either corroded silver plate or of some other white metal that has become blemished.
Examine any decorated or raised areas for signs of wear. On silver plate, these areas are usually where the thin layer of silver rubs off, revealing a base metal of a different colour underneath. With solid silver, there will be no change in colour but the details of the raised area will blur as more and more of the silver erodes with time. This softening of outlines is always a very strong indicator of silver.