I found this site to be very helpful in checking DNS records today. I simply put in the domain name, clicked "any" and could see any DNS records associated with the domain. Woo!
One client has their mail on my server but not their site, and another client has their site on the server and not their mail. I think I'm spent for a little while.
Dig web interface
Extensive webinterface to dig for doing nslookup, dns / nameserver query.