If the signal is mixed with others from different sources, the phase-reversal won't make any difference. It will only matter if it's mixed with an un-reversed copy of itself, and I can't see that happening in any normal setup.

AFAIK guitar cables are single-cored coaxial, and the design of the plugs makes it pretty well impossible for them to be miswired by mistake. It's easy enough to check them though. With a metal plug, just unscrew the casing and have a look, but if it's moulded then a battery and bulb is all the test gear you need.