I agree with Spitfire.
Anything plated over silver would be clearly marked with .925. I have done a great deal of research on the matter and cannot find any references to hallmarks of plated jewelry that read "14K S". I believe your "S" is a maker mark and your item is 14K.
There is some Swedish Silver jewelry marked with an S inside a hexagon symbol. This is less that sterling but must be at least .830 silver. This does not match your marking either since the hexagon in missing. There is also some British Silver Plate which is marked with a block S but like the Swedish Silver would simply be silver plated over a base metal and not plated gold.
These are the markings I have researched related to gold, gold plate or gold filled pieces.
14K HEP "Heavy Electro Plate" or "Hard Electro Plate"
14k G.P. Gold Plate
14KP actually is the real deal meaning "Karat Plumb" and not "Karat Plate". This is often mistaken for plated markings. The Plumb Gold standard introduced in 1978 reduced quality variance from approximately 20 parts per thousand, to 3-7 parts per thousand. Many makers put a “P” after their karat mark to indicate compliance with this higher standard.
14k G.F. "Gold Filled". This must be preceded by the karat of the gold of the outer layer. For example, an item which is gold filled and the layer is 14k gold may be marked “14k GF”. The law in the USA states that if the item is marked with the GF the layer of gold must be 1/20 by weight of the total item. In other words, if the item weighs 10 grams, the gold part must be 0.50 grams. This number may seem small but represents a fairly thick layer of 14k gold.
14k RGP "Rolled Gold Plate" Similar to gold filled, but with less gold present. The gold layer must be at least 1/40th of the total metal weight.
Y.G.F. "Yellow Gold Filled"
KARATCLAD "Karatclad is a trademark for a very thick gold electroplating process; this type of plating is about 14 times thicker than standard electroplating."
VERMEIL Pronounced VERMAY - "A product that is made from a base of sterling silver that is coated or plated on its surfaces with gold of at least 10K finess and at least 2.5 microns thick (100/1,000,000 inch). The term vermeil cannot be used if the sterling is covered with a base metal before being coated with gold unless the presence of the base metal is disclosed."
If your item is based metal with Silver, it will be clearly marked as such with Sterling or .925 unless it is a counterfiet piece made with the clear intent to defraud the buyer which I would doubt to be the case. The idea of using your detector for ID would also work. Your ring on my Infinium would indicate a clear Silver tone if it were plated gold over silver. I don't know what tones your machine gives if they are varied for gold or silver.
Put us on a photo and great going on your GOLD piece.
DaChief