Many people would probably feel it doesn't matter all that much if you get it backwards.
If you are really concerned about inverting the phase you could probably use a microphone placed next to the earpiece to measure the phase of sinusoid, though it may be easier to compare it to the same test against a "known good" pair (or at least get the left and right the same) than to figure out in isolation what it is supposed to be.
Electrically it is possible the cable is coaxial and EMI/RFI arguments might be made that the outer conductor would be better grounded, but you would probably have noticed this when stripping the cable. You might also determine it with a capacitance meter measuring each conductor relative to something outside the cable. But again, few people would worry about this.
Or you could just strip whatever is left on the plug end and see what parts of the plug the various colors correspond to.