What is the traditional way of verifying the accuracy of a crystal calibrator?

To ensure a crystal calibrator is perfectly accurate, operators traditionally tune their receiver to a known, highly precise standard frequency station, such as WWV or CHU. They then turn on the calibrator to mix its signal with the standard. By adjusting the calibrator until the heterodyne (the difference or beat note) between the standard and the calibrator drops to exactly zero Hertz, the operator achieves a "zero-beat." This proves the calibrator is precisely aligned with the atomic-clock standards of the broadcast station. Comparing the oscillator to your own receiver or transmitter is ineffective because local equipment is inherently prone to drift, making it the very equipment that needs to be checked against an absolute standard.