A/C power line polarity on new phono preamp

Getting ready to help a buddy finish setting up a new phono preamp over the weekend…he’s not sure about the A/C power line polarity (rear panel switch allows “A” to “B” switching for best performance).   Any suggestions on what’s best to listen to as you switch from A to B ?   Certain kinds of music and/or test tones best for this?   Any specific suggestions welcome.   
Thanks