Coupling between a cable and the main power line: how to measure it?
Ok, as you are not seeking for an answer but rather for an opinion, here is mine: 

I think you want to know the capacitive coupling because you might have some noise from the mains on your signal lines, right?

In your case, I would not care about capacitive coupling, because (if you have a problem) it might more be an inductive loop one. You wrote that both boards have the same ground and power supply, so there might be a big ground loop in your system setup? If yes, this is the most important thing you need to fix. 

Edit 1:

If you want to measure capacitive coupling in your setup, i propose the following circuit: Disconnect source and load and instead short the ends of the cable. Then make a measurement with the scope and you should see no coupling voltage (see explanation in my comment below) Next, connect two 50 Ohms resistors at each end of the cable. Then make a measurement of the (coupled) voltage on the cable. Why 50 Ohms? This is just a useful measurement resistor. You can also use another value. Why  both ends of the cable? Then you wont miss the inductive coupling thats also likely to happen. 

From that measurement, you can calculate the coupled current source strength.