How to know which user did reboot or shutdown when multiple desktop sessions are active?

I have a Ubuntu installation where multiple users have desktop sessions at the same time, some over XRDP and another directly on VT7. How can I find out which of these users initiated a shutdown or reboot, e.g by the XRDP mechanism or by the systray popup menu? For example: Like in Windows system in event viewer I can see which user did restart or shutdown by that the user click on button in each session even in RDP so the same I search to do for Linux system None of the users will use the command-line to perform shutdown or restart.