Wiring question

[Automission]

I'd like to ask a question. When wiring speakers, after connecting them to the speakers, how do you tell which cable on the other end is Red and which is black?
The cable I've got is sort of braided so I can't trace it along. Also, i'm using bi-amping cable for my centre speaker, so what should I do with the two unused cable ends?

I've also heard a tip to conform which is why. They say to connect the wires to your speaker, then touch the wires at the other end to the positive side of an AA battery. Whichever moves in response is positive. Is this a safe thing to do?

Will it matter if I use a thinner gauge for the rears, while I use a thicker for the fronts/centre?