Well, perhaps an easy one to sort out. First, if the only mark is “Chester Plate” it is almost certainly silver plate and not sterling. Second clue is that sterling never tarnishes, even under severe conditons, to a violet hue. On the other hand, silver plate most always goes to the violet hue when severly tarnished. The reason is that the thin electroplated silver coating on plated items is 100% pure silver vs. the 92.5% alloy of silver and copper that is sterling. The pure silver turns violet when tarnishing vs. the sterling alloy that turns black.
Since it appears you found the spoon with a metal detector, and I also hunt for stuff with one, try this test: lay out a known sterling item, say a spoon, on good bare ground, then lay your found item next to it a good distance away. Turn the gain control on the metal detector up to just detect the sterling item, then scan the dug-up spoon. Silver plated items will produce a much higher signal return than the sterling.
Finally, if still in doubt, take an “as found” picture, then polish your spoon up with any good silver polish and post it up with a good “as polished” picture showing the marks…
Thanks for asking here - we all have treasures in our cupboards in our silver, and with the price of silver now running $US 11.50 a troy oz. many of us are getting giddy with the values.