How can I tell if it is 925 sterling silver?
The tap test
Tap your piece of silver Jewellery against a coin and it should emit a high pitched ring for a second or two.
The magnet test
Sterling silver is not magnetic. See if it is attracted to a magnet.
The rub test
Silver oxidizes when exposed to oxygen. Take a white cloth and rub it. It should leave a black mark on the cloth.
The ice cube test
Silver has high thermal conductivity. Take two ice cubes, place them next to each other and place your silver ring on one of them. It will cause it to melt faster then the other one.
The bend test
I'm a bit reluctant to mention this one. I don't want you breaking your Jewellery! Sterling silver bends fairly easily. You should be able to bend it without excessive force. But like everything, it has it's limits, so be careful.