I only know a little bit about this because I got one for my father. There is a metal test--referred to as an "acid test". Also there is a way to tell from polishing but I'm not clear on it.
Again, an expert could give you a better answer. You should ask this question in the
American Pocket Watches forum.
To follow up on Jon's comments, either of these two tests are frowned upon because they are destructive, and still not definitive.
Acid testing involves scraping a sample from the case and applying an acid reagent, then comparing the reaction with a sample to determine gold content. Two problems with this are that process damages the case, and that if the sample is taken from the surface layer of a 14K gold-filled case, the test will often read as 14K, simply because the outer layer
is 14K.
Polishing tests involve either rubbing so vigorously that you remove enough metal to "prove" that it isn't plated (obviously taking away any detail, patina, or contour), or those who claim that they can tell from the "sheen" of polished gold that it is genuine, which is again, not definitive.
I've had numerous debates with sellers who "tested" cases as solid gold, in spite of manufacturer's markings to the contrary.
And with a little practice, as Jon said, the "feel" of a solid gold case, especially in larger sizes, becomes apparent. To gain this ability, you need to handle as many gold cases as you can, test their heft, and the "flex" (very carefully) of the lids, and try to do it side by side with quality gold-filled cases to "feel" the differences.
My two cents,
Cary
