some discs were inverted polarity relative to others
I only checked polarity between the reference pressings. I wanted to hear each with the same relative polarity.
As far as the mechanics, just plug a sampling scope into a tape output and check first impact. This also let me check max level and to a lesser degree compression if the wave shape was altered.
As mentioned, multitrack recording may have inconsistent polarity between tracks. I didn't record or mix the album, so I had no idea as to absolute polarity.