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Well the convention is to mark the positive (+) wire in some way - a contrasting colored stripe, a ridge in the insulation, even sometimes a different colored wire, say yellow(gold) vs white(silver). But this convention is not always followed.
@JackG, You can use a DMM to test AC voltage output at the amp speaker terminals, or speaker lead wire. The reading (signal present - Amp ON) will be a positive number if the test leads are on like polarity, or it will read a negative number if you have positive lead on a negative terminal.
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