Seeing clients authenticating against DCs

Hi,

is there any way of checking from the server, which clients have logged on
(authenticating Kerberos, etc.) I need to see if workstations are logging on
to Domain controllers which are not on the same site subnet.... we have a
very complicated DC/site structure and have historically suffered when
remote DCs have gone down. I am trying to ascertain whether we are still
likely to be affected by this.