Measure a power supply voltage with floating ground
If your grounds are guaranteed to be close, and the voltages are within the common mode range, a simple differential amplifier may be enough.

Otherwise, depending on the accuracy you need, your best bet is probably to use a linear opto-coupler circuit.



In your case the left side would be powered from the thing you are testing and the Vin would be some division of that voltage. However if the source goes below the supply requirements of U1, that will not work.