See the answer I provided in the other thread where you posted the same question: Developement Thread - app for resetting bulbs (Device suggestions welcome)
To summarize, I'd recommend an app like Device Watchdog and configure it to monitor last activity from your sensors. You'd have to have an idea of how often they tend to report in and how long you want to wait before you get notified that they aren't doing so. Protocol does not matter (whew, there, new information that wasn't in the other post 
).
