You can use the following web service to see if your DNS servers or resolvers need attention. For Windows:

nslookup -querytype=TXT -timeout=10 porttest.dns-oarc.net.
nslookup -querytype=TXT -timeout=10 porttest.dns-oarc.net. DnsServerName

For *nix:

dig +short porttest.dns-oarc.net TXT 
dig @DnsServerName +short porttest.dns-oarc.net TXT

Where DnsServerName is the name of the DNS server you want to check. You are looking for GREAT, GOOD or POOR in the results.

Original Reference