According to my crafty girlfriend, wire for jewelry crafts can be one of the following:
Stirling: This is ~92% silver alloyed with other metals to achieve desirable mechanical properties.
Plated: This has a thin silver plating on a cheap base metal like steel.
Silver Filled: This is a very thick (~50% of the wire thickness) coating on a cheap base metal.
I agree with PorcupineKate, see if a jewelry shop can test it for you. If they can't, or if you live out in the sticks, here is what I suggest:
- Use a magnet to determine if the wire has iron in it.
- Like Rob_Cleveland said, try the ice test.
- To be really sure, get ahold of a volt meter, sand the ends of the wire and use resistance measurements to calculate the resistivity of the metal. Compare this to the listed value for silver