On Ubuntu (and sometimes Debian systems) the existence of the /var/run/reboot-required file indicates whether or not a reboot is required after updates have been applied.
I have a github repository with updated versions of these PRTG sensors at: https://github.com/andygajetzki/prtg-sensors
The below script should be placed in /var/prtg/scripts on the machine you wish to monitor. Once an appropriate SSH Script Sensor is created, it will allow PRTG to monitor this parameter of your system.
#!/bin/sh if [ -f /var/run/reboot-required ] then echo '2:0:REBOOT REQUIRED' else echo '0:0:OK' fi