Many items go into the determination of whether a sensor is health or not. Perch constantly looks at metrics around disk, CPU and memory utilization.
In addition to metrics, the application is constantly considering the state of network health and status of required applications.
The sensor health page gives a high level overview of the rules that the sensor is running, alert levels as well as information into how many devices are on a network and visible to the Perch sensor.
To view sensor health, click on the name of that sensor in the sidebar.