I use a very simple, yet helpful, DNS tool: http://www.WhatsMyDNS.net
Many times, I'll make a DNS change to one of my servers or hostnames and I'll wonder how well it's propagated across the entire Internet.
In "the old days," a host name generally pointed to a single IP address no matter where in the world you were when you tried to access that domain name.
But, in today's world of redundant cloud storage, you will have a hard time locating the exact server where your web resources are located. It's much like trying to locate an electron's position and momentum in an electron cloud. I host bio.joemoreno.com on Amazon's S3, which has about a dozen different IP addresses.
While Amazon S3 guarantees at least triple redundancy, it might, actually, be better than that.
Always keep in mind when playing around with DNS updates that it's best to set the TTLs to something very short for the new DNS entries and, doubly so for negative caching TTLs. I generally keep these specific TTLs to less than 60 seconds with a generic *.example.com hostname also set to a very short time period.