If it is strongly attracted to a magnet, it's steel.
Nickle silver, which is a nickle-containg brass will be about like well, brass in hardness. It should have a very,very weak, if detectable at all affinty for a magnet. My guess is that's what the "white metal is.
BTW, there apparently is a lot of stuff out there being marketed as "sterling silver" (92.5% silver) that is stongly attracted to magnets. The majority of such items were reported to be marked .925 and were represented as orginating in Thailand or Italy. (Remember that some clasps have a steel spring in them) Something marked "STR", ".925", or "Sterling Silver" should not be attracted to a magnet. Don't believe that that a "tarnish-proof" coating will cause magnetism.