There are a lot of ways a Windows Domain Controller can act weird. Recently I had some which replied to a ping, replied to DNS requests, but failed to reply to LDAP. Since LDAP services are pretty high on my list of expectations for a Directory Services, I wanted a lightweight way to monitor LDAP. DC LDAP Ping.vbs is a script which enumerates the list of domain controllers in a domain, then binds to each in turn, measuring the time it takes to do so. You can set audible alarms, and run continuously if you like.
For more DC status, try the newer Quickly Check Domain Controller Health script.