Download PsLoggedOn. I quote:
local computer, or a remote one. If you specify a user name instead of a computer,
<b>PsLoggedOn</b> searches the computers in the network neighborhood and tells you if the user is currently logged on.
<b>PsLoggedOn</b>'s definition of a locally logged on user is one that has their profile loaded into the Registry,
so <b>PsLoggedOn</b> determines who is logged on by scanning the keys under the <b>HKEY_USERS</b> key.
For each key that has a name that is a user SID (security Identifier),
<b>PsLoggedOn</b> looks up the corresponding user name and displays it. To determine who is logged onto a computer via resource shares,
<b>PsLoggedOn</b> uses the <b>NetSessionEnum API</b>. Note that <b>PsLoggedOn</b> will show you as logged on via resource share to remote computers
that you query because a logon is required for <b>PsLoggedOn</b> to access the Registry of a remote system.
Just copy <b>PsLoggedOn</b> onto your executable path, and type <b>psloggedon</b>.
usage: <b>psloggedon \[-?\] \[-l\] \[-x\] \[\\computername | username\]</b>
<b>-?</b> Displays the supported options and the units of measurement used for output values.
<b>-l</b> Shows only local logons instead of both local and network resource logons.
<b>-x</b> Don't show logon times.
<b>\\computername</b> Specifies the name of the computer for which to list logon information.
<b>username</b> If you specify a user name <b>PsLoggedOn</b> 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.