Another easy (audible) way to tell if your speakers are out of phase:

Play a some music with a decent sustained bass line. Turn the balance control to either full left or right. Note the bass. Then center the balance. Note the bass again. If you have less bass with the balance control centered (both spkrs) then the two are "out of phase". If they are in phase then a centered balance control should result in more bass.
This also applies to front and rear speakers.

Also, Just because you have all speakers wired + to + and - to - doesn't mean where you are sitting they will all be "in phase". Sometimes it helps to "flip" the polarity of the rears. Or, a subwoofer for example, if you have a sub running off of its own amp. Some amps "invert" the signal. When its input goes positive, the amps output goes neg. In this case the sub would need to be flipped to optimize the bass.