Find out which DNS server you're using

 On, we can check /etc/resolv.conf but how do we know which IP is actually being used?

We can run dig whoami.akami.net

[source]

Similarly, we can use nslookup some_domain.com

Neither of the above solve the problem where in the case of my router, which is connected to 2 modems, each with their own DNS settings. So in my case, the DNS returned by the above commands (unless manually overridden) would be the IP of the router or default gateway.