As far as I know, there's no real direct way to do this. Every script example I've seen grabs a list of computers from AD, and then queries the computers for the last logged on user. You might consider running a script against a domain controller's security logs for a corresponding logon event. I'm away from my lab right now, but I'll see if I can whip something up. If you don't have access to a DC though, you might have to go the long way around with querying the machines.