GoForSmoke:
All voltages are relative to ground. If Arduino ground is not the same as the ground of what you would test then who knows what the either end sees?
Not true.
Many of the Atmega chips such as the 2560 (but not the 328) can measure differential voltages - the voltage between two of the analog pins. Many of the chips also have amplifiers so that they can measure very low voltages. Neither of the pins needs to be referenced to the Arduino GND. IIRC the only constraint is that the voltage between the two pins must not exceed 5v. RTFM.
Of course I am assuming that the differential voltage is quite separate from the Arduino GND - for example powering it with one battery and measuring a circuit powered by a different battery and with no common connection between the circuits.
...R