Couldn't you hook it up normally and then play the Stereophile Test CD #2 that has a track to compare in-phase and out of phase?
https://www.amazon.com/Stereophile-Test-CD-Compact-Disc/dp/B00008FUKK
https://www.discogs.com/Various-Stereophile-Test-CD2/release/3692778
https://www.amazon.com/Stereophile-Test-CD-Compact-Disc/dp/B00008FUKK
https://www.discogs.com/Various-Stereophile-Test-CD2/release/3692778