Assuming you are the admin of the system, you can monitor the log-ins/outs of the rest users from within a UI. That is boomerang and it will come back to you, if someone else is the admin ;)
Session History Logs
This new feature is under Settings->Users View, and is just one more of the dozens new features in Gnome-Control-Center 3.8.
Skoutz is my only idol. Probably because since kid, I had this simple dream. To be a millionaire ;)
There is a label that shows our last Login time and a button that keeps the entire history and displays it per week.
Of course users can only see their own information, with the exception of admins that can see everyone’s details. Aside the fun of it, this is a security fix, that helps us to monitor strange behaviors.
In GNOME 3.8 unfortunately there are still only two types of Local Accounts, Standard and Administrator, plus the Enterprise Logins for Active Directory/kerberos realms.
The plan here is to create restricted user sessions. There are some initial designs with Lockdown App, but nothing more. Built-in parental control (and other pre-defined sessions) out-of-box support should be a major goal for GNOME.
Just a nice small detail.
In case we are going to Shut Down our box while there are some users logged in, GNOME 3.8 will give us some more information than what 3.6 was doing.