well, using a resistor-devider you could have a transistor turn on at 48V and turn off at 10V...
For example, if you pick resistors to device the voltage by approx 65 then 48V would give 0.73V on the transistors base, causing it to switch.
while 10V would give 0.15V on the transistors base, so it won't switch...
Use the transistor to switch a 5 - 10V signal on and off and feed this to one of the handshaking lines of the pc's serial port, You can poll the handshaking lines at any moment...