Now that both Google Public DNS and OpenDNS offer alternative, public DNS services anyone can use instead of their service provider's DNS servers, the question is: How do you know if your DNS service isn't working properly and if you should switch?
Smarterware reader Nicholas has the answer. He says:
The easiest way to determine if your chosen DNS servers are down, you can use nslookup or dig command line tools. Open a command line prompt (Select "Start > Run" and type "cmd" on a Windows machine, "Applications > Utilities > Terminal" on the Mac) and type: dig http://www.google.com or nslookup http://www.google.com
If an IP address comes back and the web page appears to be loading properly in your browser, your DNS servers are working fine. If no address comes back, or an unexpected web page appears despite a successful dig, there is a problem. "dig" is the newer and recommended tool, but both should work fine for basic troubleshooting purposes.
A few months back I knew my DNS servers were screwy when I couldn't load certain web sites, but DownForEveryoneOrJustMe.com said those sites were up. At that point I switched my DNS service to OpenDNS and that solved the problem. Here's more on how to troubleshoot a flaky internet connection. Thanks, Nicholas!
Oz ed note: As several commenters have pointed out, one potential risk with changing your DNS servers is that you may find downloads that should be unmetered with your ISP will start being counted.
How to Know When Your DNS Servers Are Failing [Smarterware]