Gaston,
Your idea will probably work, but we need a simple, portable solution, with an LED display.
Steve,
Since speakers are very linear, for low distortion, you can't hear the difference between a pulse that is coming or going.
Since most audio amps are not DC coupled, it is very hard to distinguish the polarity of multiple uni-polar pulses.
R,
Another idea is to do what your ears do:
1) Feed a low frequency tone to both channels, like Gaston suggested.
2) Have a single amplified mic placed between the speakers, driving an LM3915 dot-bar LED driver.
3) If the speakers are in phase, the LEDs show a large signal.
4) If the speakers are out-of-phase, the sound cancels, and the LEDs show very little signal.
But you can't do that with mid-range speakers nor tweeters, because the phase changes with distance.
You could probably even do that with program music, if you turn-up the bass and turn-down the treble.