Bucking_Horn explains it nicely below:
If you have a Linux host, you could download the pihole-FTL binary with below two (for x86_64 architecture):
cd ~
wget https://ftl.pi-hole.net/master/pihole-FTL-amd64
Make it executable:
chmod +x pihole-FTL-amd64
And run below to do a discovery for IPv4 an IPv6 on your network:
sudo ./pihole-FTL-amd64 dhcp-discover
It will show IPv4 + IPv6 DNS servers advertised on your network for diagnosing.