One of my DNS diagnostic tools is a program called nsdig; it makes a query to all of the authoritative servers for a zone. This makes it convenient to both look for inconsistencies and see what the authoritative servers are saying, as opposed to whatever is filtering through your local caching servers.

As another one of my little shell scripts, and as a practical example of how little utilities stack on top of each other to everyone's benefit, here's nsdig and its component bits:

; cat nsdig
#!/bin/sh
case "$#" in
   3) nssrc=$3;;
   2) nssrc=$2;;
   *) echo usage: nsdig T WHAT [DOM] 1>&2
      exit 1;;
 esac
 for i in `nsaddrs $nssrc`; do
     echo "---- $i ----"
     dig +norecurse $1 $2 @$i
 done

(The third argument is necessary for cases like 'nsdig a www.foo.com foo.com'.)

; cat nsaddrs
#!/bin/sh
addr `sdig ns $1` | sort -u

; cat addr
#!/bin/sh
for i in "$@"; do
  sdig a $i
done

; cat sdig
#!/bin/sh
exec dig +short "$@"

(dig is the standard DNS lookup utility. Honesty compels me to admit that the version of addr that I actually use is a much more complicated Python program with various additional features that are irrelevant for nsaddr's usage.)

The one building block that is not as useful as it could be is dig, which sometimes insists on sprinkling its output with extraneous bits. (For example, even the short dig output for TXT queries has quotes around the results, which are almost always surplus.)