Determine if Server 2016 DNS is set to forward external IP address requests to public DNS servers

How can I determine if a Server 2016 DNS server is set to forward external IP address requests to public DNS servers on the internet?

An organization I consult for currently has these DNS server addresses assigned to client computers via DHCP:

10.10.10.20 (internal IP address of primary domain controller (PDC)
8.8.8.8 (secondary DNS server IP address)
8.8.4.4 (third DNS server IP)

I would like to change this DHCP DNS scope to be:

10.10.10.20 (internal IP address of primary domain controller (PDC)
10.10.10.25 (internal IP address of second Active Directory domain controller)
8.8.8.8 (third DNS server IP address)

This second set of DNS server addresses will provide better redundancy in case the primary domain controller (PDC) goes down.

But before doing this I want to test these two Server 2016 domain controller DNS servers to make sure that they will forward any external IP addresses to the public internet.

How can this type of testing or evaluation be done?