Start by downloading, installing and running namebench. On the first window, you'll see a field labeled Nameservers. This will automatically be filled with the IP address of your current DNS.
Below the Nameservers field are two checkboxes. One says "Include global DNS providers;" the other says "Include best available regional DNS services." Leave both of these checked.
The next area is for secondary options. The first checkbox lets you check if the DNS is blocking certain sites. If you're looking for a DNS with filtering options, definitely select this one. It will tell you how effective a DNS is at blocking unwanted content.
The second checkbox will publish your results anonymously. This will help provide more accurate results to you and others in the future. You can leave this blank or check it. Neither will affect the comparisons you're given.
In the Health Check Performance section, you normally want this set to Fast. This will test the speed of 40 nameservers. But if your Internet connection is slow or unreliable, change Fast to "Slow (unstable network)."