1) 99 percent of watch cases will have the karat stamped on them. If USA, then 14k 18k etc.
If german or Swiss then, /750 for 18k, .583 for 14k .333 for 8k etc. (sometimes Swiss cases will be marked with a Squirrel for 14k or a woman’s head for 18k)
If French (and a few other countries, a small hallmark, usually a bird beak of some kind or an animal of some kind for 18k. (Some early American gold cases were often 15k to 18k and were simply marked with an Eagle or a horses head or whatever.
These are bona fide American hallmarks that CAN be ID’d and are often overlooked by dealers. I have had 18s Waltham watches lately that had a simple horse or eagle mark and were solid 18k gold that weighed as much a 55 DWT! you do the math.
Make the mistake of not buying one of these and you might be making a 1700 dollar mistake! (and 70 percent of the time, American watches in a 18k cases had movements that were high grade and worth 100 to 1200 by themselves without the case!
2) Many American cases were made by companies who used odd patents, Like Waltham who had a case that was essentially gold filled but instead of brass on the inside, had 8k gold.
These are often overlooked by folks and can be had cheaply. Jack Hunt always sends them back to us for instance as “non gold”.
They test it with either acid or a torch and get an odd color underneath the outside 14k. This is 8k which DOES get a bit of bubbling with acid.
These cases are worth 150 or so in ladies watches and up to 600 dollars in mens cases. Also some early American watch companies were scammers and used extremely low karat but marked the watches with a simple “18” mark. not 18k but simply “18”. these are usually 8k to 9k gold and sometimes only the lids are 9k and the band around the watch is filled.
Next you have some German and Swiss cases that are gold on the lids, the side bands and everything but have a non gold metal for the dust cover.