If you have Dekstop version with network manager installed:
nmcli dev show | grep DNS
A better way for servers:
systemd-resolve –status
Not
cat /etc/resolv.conf will give you
nameserver 127.0.0.1
or
nameserver 127.0.0.53
because Ubuntu uses dnsmasq, resolvconf, or more recently systemd to get it’s name servers from Network Manager.