In the case of a mag, the RPM indication would go to 0 if it stopped sparking for most, but not all, reasons (the instrument senses the voltage collapse in the coils primary winding, twice per revolution). A Pmag is not that simple. If the CPU failed, RPM would go to 0, but there are many other types of failures that will prevent a spark and not result in a 0 RPM reading, such as a failed coil.
If you pay attention to your engine instruments, it should be pretty obvious when going from 2 ignition sources to 1. Power will drop 5-10% for any given fuel flow and EGTs will rise around 150 degrees from where they were with both ignitions. Had it happen once. I noticed speed was down a bit from what I expected at the given fuel flow. Looked at EGTs and immediately knew I was on one ignition.
Larry