If you need to find out what DNS you are currently using (for troubleshooting purposes, this snippet might help):
$ tcpdump udp and src port 53
$ tcpdump udp and dst port 53
$ tcpdump -n -s 1500 -i eth0 udp port 53
These 3 commands will sniff your packets for port 53 interactions and will let you know what is the ip address of the DNS server that you’re interacting with.