Instructions
Look for any marks on the piece. Solid silver should carry an assay mark to prove that it reaches a legal standard. This will be a numerical mark such as "925" or a symbol such as the "lion passant" -- a lion walking in profile -- on British silver. EPNS will have the letters "E" and "P" for "electroplate" and possibly "A1" for high quality. Most Sheffield plate will be stamped with a maker's name.
Inspect any raised decorative features on the item. Look for a blurring in some of the detail, almost as if it has melted. This is a sign of the softness of solid silver. If this isn't what you see, continue to the next step.
Inspect the piece for other signs of wear, paying particular attention to the tops of lids, edges and any other areas that might suffer from repeated polishing. To identify silver plate, look for places where the thin layer of silver has worn away, revealing the base metal underneath. If the exposed base metal is a pale yellow, then the item is EPNS. If you see a chestnut color, then you're holding a piece of Sheffield plate, which is likely to be somewhat more collectible than EPNS, although not as sought after as solid silver.