If you are interested in understanding the voice quaility in your netowrk then you can use the ART QoS reports. If you are interested in the health of the CalManager server itself then you will probably want to start by looking at the Application Event log for messages/errors gnerated by CallManager.
You can also use Performance Monitor to many areas in CallManager. There are many CallManager related counters. For example, you can monitor Transcoder resources for total available, currently active and failed resource requests.
