Of course, the classic method is to use a TDR (time domain reflectometer). IIRC, procedure is as follows. Terminate one end of the cable with a known, calibrated resistance, send a short pulse down the cable and look for (or better still, measure) the magnitude of any return pulse. If zero return, the Zo = that terminating resistance.
Without thinking too deeply about that, it may be possible to build a relatively simple piece of test gear to do that. Any ideas, anyone?
Al.