Testing a power chain with a microcontroler
Aside from the comment of Icy if can think of some ways to monitor the voltage drop on the diodes: 


you can do this with a simple differential OPA circuit and interface it's output to an ADC of the microcontroller. Or you use a differential ADC input (using two input lines) if the used ADCs frontend supports this feature.
if you do not have a differential ADC input but the voltage drop on the diode is high enough compared to the resulution of the ADC (e.g. in your microcontroller) you just measure the voltage before and behind the diode with two ADC channels and evaluate the difference by software. 
you build a simple comparator and interface this to a logic input of your microcontroller (diode good / diode failed signal). Examples and a pretty good tutorial for comparator circuits is found here: Op-amp Comparator Tutorial


A completely different approach would be to monitor the current drawn from each power supply. Just use a reasonable dimensioned measurement shunt resistor and monitor the voltage drop over it. Again a differential OPA circuit or a differential ADC input channel can be used.