If it's marked with the word silver and the fineness, it has to contain at least that much silver by weight.

The early silver premium tokens (silver strikes) didn't exactly follow the regulation as their fineness and the word silver was marked on the brass ring that contained no silver at all. A complaint was made by the fellow who produced the first silver strike catalog (what was his name?) and I believe the makers moved the marking location to the center silver part, but it took them several years.