You can actually calculate the speed from just the physical parameters of the cable; spacing of conductors, materials, etc.
It is very common to measure the delay of a signal through some circuit. If you know the length of the circuit and the time of the delay it is easy to figure the speed.
example: Get a length of the cable with the output hooked to an oscilloscope. Inject a pulse into one end of the cable that also triggers the scope with a very short lead. Measure the time it takes for the pulse to get to the output of the cable relative to the trigger. Do the math.
My area of expertise is not in measuring such delays but I suppose there is test equipment specifically designed to do it. Google "transmission lines" for more info.
It is very common to measure the delay of a signal through some circuit. If you know the length of the circuit and the time of the delay it is easy to figure the speed.
example: Get a length of the cable with the output hooked to an oscilloscope. Inject a pulse into one end of the cable that also triggers the scope with a very short lead. Measure the time it takes for the pulse to get to the output of the cable relative to the trigger. Do the math.
My area of expertise is not in measuring such delays but I suppose there is test equipment specifically designed to do it. Google "transmission lines" for more info.