This should be possible but you will need to write a custom SQL query alert.
So there are a couple layers to this, it is pretty easy to trigger on nodes that have no custom pollers assigned to them. So if you wanted to set something up like notify my whenever an APC UPS has no custom pollers on it then I would do it this way.
Go to a new alert and set it up like so:
You can get as fancy as you want with the criteria, for example wanting to make sure all your ASA Firewalls had some kind of custom pollers on them would be something like vendor like 'cisco' and machinetype like '%ASA%', or you could set up some kind of custom property if that would help
So like you said, not having any pollers is a good indicator that something is wrong. Unfortunately the way those tables come together if there is any poller assigned to it then this won't trigger and it gets a lot more complicated to try and do an audit of the pollers that are assigned to the devices to make sure they are all the correct ones. In that case I would probably be trying to make a report that just lists off all the pollers assigned to each node, grouped by machinetype and then by node and just look that over once in a while.
Hope that helps
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