You can also check visualy if the channels have flipped phase. This procedure does not apply to patterns with negative lobe, eg. fig 8 or hypers. Find a short, independent clap, which is noticeable on the right and left channels. If the signal rises in the right and left channels, record should be in phase. If the signal in one channel rises and in the second goes down, the recording is out of phase.
With crossed figure-8's (Blumlein) you can identify what quadrant the sound arrives from using that method. The transients will first swing positive in both channels for sounds arriving from the front quadrant, negative in both channels for sounds arriving from the rear, positive in the left and negative on the left channel for sounds from the left quadrant, etc. Blumlein has equal sensitivity to sounds arriving from all horizontal directions. Only level and the polarity relationship between channels changes with direction.
I hope I use the correct terminology. Phase shift is a time delay of one channel over the other. It has an effect on the stereo image. Out of phase is flipped phase (flipped polarity) but no delay.
That terminology is popularly accepted. Even though technically incorrect, it's common to refer to
polarity inversion as
phase-inversion or
flipped-phase. I always try to use the term
polarity when referring to a positive/negative inversion (no time component, same inversion across all frequencies), and
phase when referring to waveform alignment which tends to involve time delays and vary with frequency. And it bugs me to see polarity inversion switches on gear labeled
phase instead of
polarity, due simply to widespread popular acceptance of the technically incorrect term. Not too hard to print the correct word on label. People will figure it out and that avoids potential confusion.
Note that in some special cases a true
phase shift can have the same degree of shift across all frequencies and/or no time shift involved, but those are not the common phase-shifts we deal with ordinarily. I won't go further into that unless someone is interested.