The monitoring servers monitor each other (and themselves), they should be in different data centers. You can also use a third-party service to monitor parts of your infrastructure, including the monitoring server. Depending on your needs a simple service like Pingdom could be used.

If you are wondering how to monitor if both data centers go don't at the same time, I'd say for most companies you don't worry about it. 1) The odds are extremely low. 2) You will notice if two DCs go down. 3) That nightly email that says "I'm up" isn't going to help here. 4) Even the free version of Pingdom will alert you when your whole datacenter is down.

There are all sorts of other things to consider with redundant monitoring, but that's the job of a sysadmin -- identifying failure points, assessing risk, etc.