As a guy who's concerned with phase perhaps more than most, maybe I can offer some overblown advice.
First, never assume anything. The only way to check phase along the electrical path is with a signal generator and a scope, and the only worthwhile end points are at the source (output of the signal generator) and at the power amp outputs, so any inversion cancellation along the chain just washes out. That way you can just swap your speaker leads to correct an issue. Note that you probably need at least one differential (floating) probe to do this, since it's usually a bad idea to connect preamp ground to speaker terminal negative (which generally floats), and while a scope's probe ends are isolated, the ground clips are tied together internally. And note that this test, comparing phase on channel A and B of your scope, is useful only on equipment with negligible latency (e.g. analog). But there's a way out. More on that below.
Second though, a mike test is in order for the speakers. Once you've tested the woofer with the battery test mentioned before, you can check the polarity of the mike (and its preamp, if it's a dynamic mike) by comparing your test signal against the mike output, again on the scope. Ah, but now, with speakers, mikes and the distance between them, we've introduced latency. That's why a continuous sine wave is not optimal. You need tone bursts.
Third, it's been mentioned that some speakers invert phase between drivers to compensate for phase cancellation, probably a result of physical offset between voice coils. Note that this isn't putting them "in phase", it's just putting them out of phase enough to create (some) addition with a continuous sine input. To see the real story of how off they are in timing, repeated short tone bursts (with a good size gap of silence) are required. Burst generators aren't easy to come by for some reason, but any computer (laptop, e.g.) can be fitted with Audacity for free. You can generate a sine of given frequency and duration, then edit it to form a short burst with leading and trailing silence, then play it back in a loop to repeat so your scope won't have to trigger on a single burst. Now you can see whether your tone burst (hence your music) is inverted or not, plus you can check the phase of your drivers AND the delay between them A tone burst at the crossover frequency between two drivers should play through both and superimpose (add) perfectly. You may be in for a shock.
Which gets us here. If you are concerned enough with this (as I am), you employ linear-phase FIR filtering for your crossovers and equalizers. Now you know your filters aren't causing phase anomalies. And you separately amplify each driver. Any cheap desktop with a soundcard and free software will make a brilliant DSP for these purposes, and even a $50 pcie 7.1 'surround' sound card/DAC will make a superb multichannel (up to 7 discrete) line-level output. It goes without saying that in a multiamp setup (or any multi-driver setup for that matter) phase in all its aspects is very important. BTW, another advantage of such setups is the fact that you can perfectly phase your drivers (make them simultaneous) at the crossover points by simply programming delay to compensate for their physical distances, whether you use linear-phase filters or not. Which brings us back to latency. Computers and DSPs have lots of it, so you can't just A/B your input and output on a scope. But we no longer need to scope our input when using tone bursts. Audacity (e.g.) shows you the exact form of your tone burst. Now we're just analyzing output. As for the recorded phasing of actual source material, there's nothing you can do about that so forget it.
And even if you're not keen on computers, DSPs and linear phase filters, you still can make your tests with the battery, laptop, scope and mike to test with tone bursts for inversion on all your drivers. You'd be amazed (and perhaps somewhat depressed) at what I've found phasewise on a variety of equipment. I trust absolutely nothing now regarding its phasing.
Hint: try playing a tone burst at the crossover frequency of your drivers (say, mid/tweeter) and see how far off they are. You'll see why I went to DSPs.
