I understand that analogue interconnects are nominally 50ohm and Digital 75ohm. Leaving aside the problems that differing terminations can introduce, how do we know that what we are using is the correct impedance? You can't measure it with a multimeter and not everyone has an Oscilloscope, or if they do (like me) have never bothered to learn how to use it  (it is only a cheap Velleman one, but I ought really to try)

I have made several "whacky" interconnects over the years just to see what they could do. I am currently using as a Digital cable one that uses a twisted pair of single wire Cat5 cable for the signal and return. It works and carries the signal. Should I be using it? Is it 75ohm? Does it really matter.