Are your preferred DNS servers working?

Here's a small post which addresses this questions and provides some useful CLI stuff.
In particular, typing "dig google.com" gives this output

$ dig google.com

; <<>> DiG 9.5.0-P1 <<>> google.com
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13450
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;google.com. IN A

;; ANSWER SECTION:
google.com. 296 IN A 72.14.207.99
google.com. 296 IN A 64.233.187.99
google.com. 296 IN A 64.233.167.99

;; Query time: 65 msec
;; SERVER: 208.67.222.222#53(208.67.222.222)
;; WHEN: Thu Aug 7 23:52:16 2008
;; MSG SIZE rcvd: 76

The DNS server in use is shown in the last block.