If your using the RCA inputs Im pretty sure that the 2 outputs will both be in phase on your amp. You run into trouble with balanced amps and amps with transformers: OTL's are usually pretty tame for phase. The bijou does invert phase, but since it does the same to both channels its not important. You run into real trouble when one channel is in phase, and one is out.
Since both channels are probably in phase by default when using the single ended inputs, Just check that the right XLRs are hooked up to the right boards. plug in only one XLR, if sound comes out of only one channel your good.
If you realllllly want to check it out there is a way, but its probably easier to just listen.
I would start by just tracing the signal from the jacks through the amp boards and out with the amp off, and after discharging the caps... Use a DMM set to ohms for this - nice and quick.
After that, or with an amp with transformers:
Connect 2 100 ohm resistors together. Love the alligator jumpers.
Connect the ends of the resistors to the SE outputs of the amp.
Play a mono sine wave into the RCA jacks. Best way is to just use one output from your source and use a y-spliter to both inputs. I have 440hz saved on a CD just for this.
Turn up the volume until you have about 0.5vac from either (both!) end of the resistors to ground.
Measure ac volts from the CENTER of the resistors to ground. If its about 0.5vac, your good. if its ~0vac your out of phase.