Figuring out the polarity of a car stereo speaker is not a difficult thing to do, but you must have this information before you begin trying to hook the speakers up in your car. The most common method for figuring this out is most often referred to as the "Pop Test".
Step 1 - Mark the Wires
Before you disconnect the speaker, be sure to mark the wire, as well as it's corresponding wire on the speaker with a marker.
Step 2 - Disconnect the Speaker
Disconnect the speaker from the vehicle and place it on a flat work surface with the front side of the speaker facing up.
Step 3 - Test the Wires
Touch the positive side of the battery to the wire terminal that you made a mark on and the negative side of the battery to the other side. Don't hold the wires to the battery for more than a second, otherwise you could damage the speaker.
Step 4 - Watch for Movement
As you touch the wires to the battery, notice which direction the speaker cone (or rings of the sub-woofer) moves in. If the speaker cone pops out, then the polarity is correct. If it moves inward or down toward the work surface, then the polarity is incorrect. To remedy this simply switch the wires around for the correct placement.