Find out which DC a system has used to log in
Determine which DC a system has used to log in - Tip2
A previous user tip mentioned you can determine which domain controller a system has used to log in by typing: Start --> Run and %logonserver% NETLOGON. But instead of that, you can just go to a cmd prompt and type "set" (no quotes). Logonserver will be listed near the top (among other useful environmental information).