"Guaranteed" and "Warranted" Cases
Gold-filled cases were often marked with a guarantee, another innovation credited to James Boss, which specified a number of years that the case was guaranteed to wear. A case that's marked "14K Warranted 20 Years" meant that the gold-filled case was made with a layer of 14K gold, and was guaranteed that the gold-layer would not wear through to the brass for a period of 20 years. If your case is marked "Warranted 20 Years" or "10 Year Guaranteed" or any other reference to a number of years or guarantee, then that is a sure indication that it is a gold-filled or gold-plated case. Note that the year-guarantee was related to the thickness of the gold layer, not to the karat-quality of the gold used in the gold layers. In general, a longer guarantee implied a thicker layer of gold. Most gold-filled cases were made with 10K or 14K gold.
The year-guarantee markings on cases continued until 1924, when the practice was prohibited by law due to the failure of some manufacturers to stand behind the so-called guarantees. If your watch case says "Guaranteed for x Years" you know that it was made prior to 1924. After 1924, gold-filled cases were simply marked "Gold-Filled" as seen below.
Nickel Alloy Cases
In addition to gold and gold-filled cases, manufacturers produced cases from many other materials as well. Nickel-alloy cases look similar to silver, and were produced under many names by American case manufacturers. Each manufacturer had their own unique name, and their own formulation, but cases were usually a mixture of Nickel (45%), Copper (54%) and Manganese (1%).
Manufacturers often chose names for their nickel-alloy cases which were suggestive of silver content, but which actually contained no silver.