What you're listening for the the tonal balance, not the overall volume. If both speakers are connected properly, then bass and treble will increase volume equally when both speakers are playing. If one is reversed, then the deepest bass frequencies will not increase when both are playing, in fact, they'll go down a bit. The midbass, midrange and treble will increase but not the real deep bass.

Isolate one speaker at a time. For example, first have the faders set forward, and compare left and right. Once you know that's right, set fader for full rear and do the same process. After you've verified the rears, do the same process front-to-back to verify the front pair and rear pair are phased properly.

This only works for bass frequencies. The reflections inside the car and the number of sound sources make interference very complex above midrange frequencies, so polarity reversals aren't immediately obvious at high frequencies. So this process won't work to verify your tweeters.