We recently had a customer who needed an update to DNS, and it was taking a very, very long time to propagate. While researching how to help fix it, I found these sites that may come in handy.
To see what’s going on with DNS propagation, use this tool: What Is My DNS
And if you ever need to check a particular DNS server’s entries, use this: Query a DNS Server
Plus a very handy page describing how to use nslookup: 10 Most Used nslookup Commands