I would simply use a divide by ten oscilloscope probe
connected to the plate of the S-108 local oscillator tube, feeding
a frequency counter. The divide by ten probe provides isolation,
to avoid pulling the oscillator off frequency.

If you just want a 100 KHz calibrator, I recommend an old Bud Radio
model "FCC-90 B". It has it's own power supply, and requires
no connection to the receiver. (Just uses a loosely coupled
antenna wire near the receiver input). It has two tubes,
a 35W4 and a 50C5, in a transformerless circuit.
The older Bud model FCC-90 used octal tubes.

73,
Ed Knobloch

Allen McBroom wrote:
I've had an old Hallicrafter's S-108 since the early 70's, and trying
to tell what freq I'm on is impossible. Does anyone know of a method for
installing a crystal oscillator on this old tube rcvr, or some other method
of acurately determining which frequency the radio is receiving at the
moment?
Many thanks for the help!