All with the same even-toned subject, shoot one frame with flash on "Auto" (everything set carefully correctly, and values chosen to be well within the flash min and max distances, and at least 3 stops underexposure for the available light) for a reference exposure (can also add an available-light exposure as a second reference, even if it is not sharp). Then shoot a TTL
flash exposure with no compensation, and another with +1 stop compensation.
Theas should be on B&W film (the type that gives problems with TTL flash
accuracy) so the frames can be contact printed together to see which match.