Identifying the polarity of your speaker wire is vitally important when installing stereo equipment. Whether the speaker for a car or a home, there are dozens of ways to test the polarity of the wire. Testing polarity involves disconnecting the wire, hooking it up to an external power source and determining which wire is positive and which is negative. Knowing which end is positive and negative allows you to phase your speakers correctly. Out-of-phase speakers have a tendency to distort, and they aren't nearly as reliable as speakers that are properly phased.
Disconnect the speaker wire from the speakers.
Attach the end you believe to be the positive end to the positive terminal of a test light.
Attach the end you believe to be the negative end to the negative terminal of a test light.
Watch the light bulb. If it lights, you have the correct polarity. If it doesn't, reverse the wires.