(A follow up to similar question for 12.04.)
Prior to Ubuntu 12.04, you may see the active DNS in
/etc/resolv.conf. In Ubuntu 12.04, NetworkManager no longer works with the file. You have to directly consult the command line tool
nm-tool is no longer installed by default in 14.04 and later. Although you may still install through
apt-get install, you can't assume all Ubuntu to have that out of the box.
So the question remains. How do you know, by default installation, the DNS you're using by command line?