With a ground or reference, suppose you had 3, 6 and 12 Volts. Then it's easy. Use a voltmeter as the receiver.
No ground or reference, it gets a bit harder.
Suppose you had 3 V, and 9
So you look for the wires with a 3V positive difference and confirm the 9V one.
i.e.
AtoB has 3V
A to C has 9.
A(0),,,,,,,,,,B(3),,,,,,,,C(9)
bad example number wise, but you can't mistake the reference in this case.
Voltage like 1, and 5 with a 9V supply and a constant current source,