Do you mean make a test circuit to say whether the voltage is either too high or too low for the front panel? (48v O.K. 49v = fault )
I made a circuit picture below, is this what you mean? i would have to breadboard to find the resistor values as i have not used a zener before. i presume a 47v zener might work for both 48v and 49v and above detection.
That is the idea. I figure as long as the power LED is running off of the output, it might as well indicate something more than *some* voltage is present. I might put a real power indicator on the input as well.
If the LED forward voltage drop is 2 volts, and there is 3 volts across the current limiting resistor, then the zener diode should be 48-2-3=43 volts which is the standard value below 47 volts and for 5 milliamps, the current limiting resistor is 2V/5mA=600 ohms. 560 or 680 ohms would work fine.
Actually when taking into account the standard zener diode tolerance of 5%, a 39 volt zener diode should be used with a roughly 1.4k series resistor.
I would not bother indicating high output voltage. What I might do if I wanted to protect a sensitive or expensive load is add an SCR crowbar to the output which triggers under conditions of high voltage and has its own fault indicating LED. This would work well in combination with foldback current limiting.