In general, you should be wary of any markings on silver. Look for where the silver has worn away. Sometimes you're not lucky enough to find an electroplate marking. There's another clue to look for. Take a close look at the piece, especially the base, and look for areas where the silver has been worn away.
Some people mistakenly refer to all silver plate as Sheffield plate.
Sheffield plate is a process that was invented in England about 250 years ago. Sheets of silver and sheets of copper were hammered together until the two became fused. The fused metal was shaped and formed into objects in the same way that silver objects were made. Electroplating, the same process used today, was developed in the middle of the last century. A chracteristic of Sheffield plate is its pleasing, warm color.