This is easy to identify if you use Firefox as your internet browser. At the bottom is a status line that tells you how it's progressing with rendering each page. Most of the time when you look it just says 'Done' because it's finished rendering. However, when it's waiting for a DNS request you'll see, for example, 'Looking up cableforum.co.uk'. If the DNS request fails then that's what you see for ages until it finally gives up and says it can't find the page.
This may just be a packet loss issue rather than an actual DNS issue. This is because this service uses UDP datagrams. Unlike TCP, this has no means of error recovery. If a packet gets lost then it's up to the application to recover - or not!
http://en.wikipedia.org/wiki/Domain_Name_System
http://en.wikipedia.org/wiki/User_Datagram_Protocol