JSI Tip 4712. PsLoggedOn freeware tells you who is logged onto a computer, or it can locate a user.

Download PsLoggedOn. I quote:

PsLoggedOn is an applet that displays both the locally logged on users and users logged on via resources for either the
 local computer, or a remote one. If you specify a user name instead of a computer,
 PsLoggedOn searches the computers in the network neighborhood and tells you if the user is currently logged on.

PsLoggedOn's definition of a locally logged on user is one that has their profile loaded into the Registry,
 so PsLoggedOn determines who is logged on by scanning the keys under the HKEY_USERS key.
 For each key that has a name that is a user SID (security Identifier),
 PsLoggedOn looks up the corresponding user name and displays it. To determine who is logged onto a computer via resource shares,
 PsLoggedOn uses the NetSessionEnum API. Note that PsLoggedOn will show you as logged on via resource share to remote computers
 that you query because a logon is required for PsLoggedOn to access the Registry of a remote system. 

Just copy PsLoggedOn onto your executable path, and type psloggedon. 

usage: psloggedon \[-?\] \[-l\] \[-x\] \[\\computername | username\]
 -? Displays the supported options and the units of measurement used for output values. 
 -l Shows only local logons instead of both local and network resource logons. 
 -x Don't show logon times. 
 \\computername Specifies the name of the computer for which to list logon information. 
 username If you specify a user name PsLoggedOn searches the network for computers to which that user is logged on.
          This is useful if you want to ensure that a particular user is not logged on when you are about to change their
          user profile configuration.