Check for /var/run/reboot-required (ubuntu/debian) and show if reboot is required.
You can install "needrestart" packet, it will inform you when it will be necessary.
Is there a plugin that shows the result of needrestart?
I don't think, I always use it on CLI.
I have created a plugin for that.
Maybe you want to test it? Please leave a comment on the github pull request, if there are any problems.
Customer support service by UserEcho