a simple "psloggedon account" identifies all computers the given account is currently logged on (and part of the same domain you start this command).
this tool is part of sysinternals huge amount of useful tools and command line helpers which helps to answer or identify common maintenance issues, like
- does my application doesn't close all used handles
- which resources (filesystem, registry, ...) does my application access
- capture of debug output (traces using OutputDebugString, DbgPrint) without using a debugger