Specifically:
Expand the wrench icon for more details on the network device in question (they must be done individually).
Select the Notify Me option. By default it is Never. You likely want Always (it will notify you each time the device becomes inactive or active) or the When I'm Away option (it will notify you also, but only when your Almond says you are Away).
Another option you might want to change is the Inactive grace period. By default it is 6 minutes. This is not too long, but if you want the notifications regarding when a device becomes inactive (drops off the network) as quickly as possible you should probably change this to the minimum of 1 minute.