It involves testing and the fineness of the metal and then stamping it with control marks that show the results.
Hallmarking is carried out by an organisation independent of the manufacturer of the item.
If you can find a copy of Tardy's International Hallmarks on Silver, you will have a better idea of what I'm talking about. US silver companies such as Gorham and Tiffany often used both marks in the late 1800's.
Britain always used the standard 925 and had another standard which is 956 silver which was called Britannia silver (this Britannia silver is seldom seen) and instead of the Lion rampant or lion Pageant you would see Britaina. Hence why British silver is sought after pre-1900 hundreds.
On this page there is a brief description of a number of different types of hallmarks that you are likely to find in a watch case, and then for the British and Swiss marks there are links to take you to the full page of information for that type of mark.
If you want to get a book about English hallmarking, Bradbury's Book of Hallmarks published by the Sheffield Assay Office is a long established reference.
In some countries, the testing of silver objects and marking of purity is controlled by a national assayer's office.
There is a lot of information on this page and I know it can be difficult to take it all in, so if you are struggling to understand the marks in your watch case please ask me for help via my contact me page.
The following list is compiled from emails of Silver Forum subscribers: The list consists of designers and maker's marks that have been difficult to find in reference materials so far.
Britain would not accept any standard below 925 as silver. Scandinavian countries used 830s silver like Denmark moved to using 925 silver in 1927 however even though a higher grade of silver was used by most jewellers in Scandinavia, they stuck to stamping there jewellery 830s as they did not have to pay a tariff to the assaying office for the change over to 925.
So most Jewellery made by fine houses in Scandinavia will in fact be marked 830s but will have a standard silver of 925.