Not all silver is marked. Markings are nice but you should do a bit of research. 925 and sterling are silver. Other markings that may include silver in the name sometimes contain no silver (nickel silver).
If you grind/sand a spot you may see copper, but if it is silver color under the top layer it is not always silver. Example - US nickel (coin) is 75% copper 25% nickel and doesn't look like copper.
Nitric acid is used to test unmarked silver. You can make it or buy it (research).
If you have a good scale you can perform a specific gravity test. This test allow you to determine the density of the item then compare that value to the know value of aluminum (2.7) copper (8.9), silver (10.5), gold (19) etc. I will provide more info if it is of use to anyone.
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