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.