If you've ever seen them test for gold, they generally just rub the link against a very fine sandpaper type of paper and then use different types or grades of liquid (I believe acids) to see which makes the gold disappear. By seeing which one dissolves the gold, they can tell the carat.
So, if they don't get under the gold plate, they won't know anything about the total gold content. That would be done by weighing it vs. water displacement.
Or maybe some fancier new methods, of course. I'm certainly not a jeweler or an expert.
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