"Sigh"... We just had a question like this not too long ago...
Well, to repeat myself, you can do a netbios lookup on their computer using nbtstat in the hopes you find some useful info.
nbtstat -A <IP Address>
Ex:
nbtstat -A 192.168.1.3
NetBIOS Remote Machine Name Table
Name Type Status
---------------------------------------------
MASTER <00> UNIQUE Registered
NETWORK <00> GROUP Registered
MASTER <20> UNIQUE Registered
MASTER <03> UNIQUE Registered
NETWORK <1E> GROUP Registered
DAVE <03> UNIQUE Registered
NETWORK <1D> UNIQUE Registered
..__MSBROWSE__.<01> GROUP Registered
MAC Address = 00-04-5A-71-7B-7E
My network workgroup is called 'NETWORK' and my computer's name is 'MASTER' and the user currently logged in is 'DAVE'. And now I know my real name! w00t.