Isolated voltage and current measurement
Simple voltage OK indication
Use a comparitor on each PSU to drive an opto-isolator if voltage is OK.
Analog voltage reading
Use a voltage to frequency circuit or voltage to pulse width circuit to drive the opto isolator.
Generating the PWM signals is easy using the LTC6992-1.

Figure 1. LTC6992-1 pinout and waveforms.
As can be seen, the configuration is simplicity itself. You need to use a potential divider to bring the expected voltage range down to 1 V max. Your pulse width will now be proportional to the voltage. Your receiver will need to measure the pulse width and the periodic time. The voltage can be calculated by the ratio of width to period.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 2. Potential divider, PWM generator and opto-coupler.

In either case your controller can monitor the opto-transistor to deduce the status of each power rail.