Gold Plated Questions For You....

Gold plating on coins is so very thin that it makes a completely negligible difference to the amount of gold used in the plating, be it 14K or 24 K.
Don't use an acid test, and risk damaging the plating.

Gold plating is just a frivolous novelty unless it was put there deliberately by the mint, to be a feature on the coin.
The Royal Australian Mint has produced quite a few selectively gold plated silver NCLT issues for the collector.

I have a ancient Roman gold plated silver coin. The gold plating has made the coin almost valueless from a numismatic point of view, despite the fact that it looks quite attractive. It was gold plated to look like a double aureus (binio), a denomination that is excessively rare in Roman coinage.

Gold plating is cheesey, period. But it looks sort of alright when the item is brand new. Still not worth it; you couldn't pay me to gold plate my silver.

As to the original question: you'd just have to take their word for it. If you MUST (as in you can't resist) acid test your coin don't be putting the acid directly onto your coin, get a touchstone on which to scratch it, and only on the edge or otherwise non-obvious place. The scratch's color will be the color of the actual metal and not the gold plating, so you don't even need to acid test it because you'll be taking off more than negligible amount to test between 14 and 24 karat, so just take their word for it because it's quite a small amount anyways, the difference between 58 cents and 1 dollar.