Silver is not magnetic. However, testing this way has its issues as some underlying metals used in the formation of the product and then are coated with silver can also make the item seem like solid silver. The test should also include other tests to make better comparisons i.e an Ice test. This is where a cube of ice is placed on the silver item (not easy on jewelry and coins) silver has the highest thermal conductivity of any common metal or alloy, though copper is right behind. The ice, if it is silver will start to melt very quickly as silver absorbers room heat quickly
This very brief question is too obscure and needs more information.
Take it to a jeweler and have it valued.
The base metal will begin appearing as the gold plate wears away. Therefore, the colour will change over time, especially if the chain is frequently handled, such as a watch chain, or a security chain attached to your house keys.