Opnsense has a traffic capture feature in the interface diagnostics menu, if you want to spot check what servers the devices are currently talking to.

Should be pretty obvious: client devices and internal services will have no traffic >95% of the time, just NTP for timekeeping, DHCP lease renewal, and associated ARP (running total: two dozen packets if you monitor them for a full 24h), then any system updaters (readily identifiable by the initial DNS requests), and finally of course you'll see the traffic of the service that the device hosts, if any, which can be easily dismissed by not looking at incoming connections (scraping uses outgoing connections)