Identifying positive and negative speaker wires
Note that the wire doesn't care which is positive and which is negative. Nor do the speakers, really; what matters is that both/all speakers be in phase with each other. 

So if the same side of the speaker wire is always hooked to the same terminal at the signal source (amp), and the same side is always hooked to the same terminal of the speakers (even if all the speakers are backwards -- as long as it is all of them), everything will work properly.

(I usually adopt some mnemonic such as "ridged to red", to help remember which convention I've chosen.)