What is it that you're trying to accomplish? Do you really need to backup this server during business hours? It would be much better to schedule this outside of working hours. Your goal as a system administrator should be to ensure that services are always available when people need to be working. I encourage you to look at backup solutions that don't require the server to be shut down at all.
It sounds like both of your servers are domain controllers, is that correct? And because you talk about the 2008 server being "off", it sounds like you're talking about making a backup by copying the VMDK file. If that's true, BE CAREFUL! This approach is risky. For more details, see the question: Can windows domain controller be virtualized?.
As to your actual question, it is possible to find out which users/computers have an active SMB session with your server, but I'm not sure if it's possible or easy to determine which computers are using the server as their Logon Server. You need to realize that checking either of those conditions alone doesn't give you enough information to know that the server is unused. You can be authenticated with one server and accessing files on another.
Please reconsider your approach, and look for a better way to backup your server(s).
