You could do this with a data collector set. This is done by creating the user-defined data collector set manually (not with a template), selecting Performance Counter Alert, and then setting the threshold for the active sessions. You can then set a task for the alert to notify you with a basic intro to criminology
or log the event to a file. Although you can get some session-level information from windows 8 updates but wont install Remote Desktop Services Manager, wknt performance object called Terminal Services Session provides quite a bit more insall.