How do I find out which remote desktop sessions are active?

The commandline way, which you can even remotely use, is

qwinsta /SERVER:{servername}

which will list the current sessions and their status, and with

rwinsta /SERVER:{servername} {sessionid}

and the proper privileges you can disconnect a session, particularly useful when the limit has been reached and you are not connected.

{sessionid} is the ID number listed in qwinsta's result

Start -> Administrative tools -> Terminal Services Manager

Click on your servers name and it will list the active sessions on the right, there is also a field for the session state. Another tab called 'Sessions' will show active users, listeners and the console sessions.

If you have a remote desktop open to the server in question, you can see other sessions on the same server in the Task Manager (Ctrl-Shift-Esc).