You could keep an array of 50 unsigned long elements and store millis in the appropriate cell whenever the sensor sends a reading. Check for any that have too long an interval between millis and the stored time. Alternatively, create another web page that queries the database, displays the IDs of the non-responsive nodes and the mega can parse them out. What are you going to do when you detect a failure?
I had an array in mind as well, but thought there might be a better way of doing it. Can an array be too large?
The web query is an interesting idea as well, I will look into how I can incorporate that.
The failure would alert several users by e-mail, same as what would happen if a temperature is found out of spec. I may also look into SMS alerts.
The sensors will be in a laboratory - monitoring cold rooms, freezers, incubators and the like. Right now, analog chart recorders are being used which are pretty much useless, some have audible alarms - which are useless if a failure happens at night, and only the cold-rooms temperatures are tied to the alarm system that would alert 24/7.
Thanks for the input!