You would use a spectum analyser with a frequency sweep function / VNA to catagorise phase and gain of the cable across the desired frequency range, and depending on the equipment either directly or with a distortion meter measure the distortion added to the signal,
Next up shielding, build something that can pulse out large EMI spikes at broadband frequencies, measure the spectrum on the wire with the other end connected to a dummy load of the standard speaker resistances (not a speaker but a resistor) this is common mode immunity,
And finally if its multiple pairs in the same wire, inject those pulses down one pair while testing the other, this is crosstalk immunity,
There are a few more things you can worm out, but the VNA option gives you impedance, phase and gain (loss) across a frequency band of your choosing, which i would say is one of the most definitive ways to describe a cable,