If you are talking about a remote user using VPN (RRAS) you can use Routing and remote service located in the administrative tools to see what IP the user has been given. Under the server name look for remote access clients, then double-click the users login name or right-click and select status.
Then using a command prompt type in nslookup and hit enter, then type in the IP, that will give you the name and FQDN of the workstation using that IP.