Local Security Policy, turn on Auditing, Audit Logon Events, Success and Failure. For a quick check, look at Computer Management, Shared Folders, Sessions...that will show you any active netbios connections to your server.
IIS, enable logging, check log...winnt\system32\logfiles\w3svc(#)\ex(date).log
DHCP, open DHCP manager, check active leases under appropriate scope.
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