This is a common problem with all sensors. Unfortunately there are several failues that a sensor can experience. To test all aspects of a sensor, create a custom signature tha twill fire on any traffic with a summary (so you only get an alert every X min). Then feed this event (SDEE or Syslog) into a system that looks for the absence of the event.
We call it a heartbeat sig. Cisco borrowed the idea and was going to put it into 6.0 as a standard signature, but for some reason abdoned the feature.
