The Remote Desktop Connection Manager has a cool feature that let’s you right-click a server and list all the current sessions. It will also let you know if the sessions are active or disconnected. This works great, unless you get the error message: Unable to Enumerate Sessions.
In order to enumerate sessions, the account you launched remote desktop connection manager with must have administrative rights on the destination server. You can either add your logged in account to the local administrator group on the destination server, or launch the manager as a user who does have administrative rights.
If you are not using the remote desktop connection manager, I highly recommend it for anyone who needs to manage more than a few servers. It was updated in Nov 2014 and works very well on Windowx 8.x and higher or Server 2012 and higher.
You can read about and download it from technet here:
Download Remote Desktop Connection Manager