In command prompt (Start > Run > Cmd), enter the following line (replace the DNSIP with the IP address of your DNS server) then hit enter:
Ping –t DNSIP
If you have multiple DNS servers then you'll need to open multiple command prompts and enter that string for each
From this point on if you see quote marks, you need the text inside the quote marks, italics text is something you choose or need to find:
If you want to save the results to somewhere then you can add " >PingLog.txt" (you will need to change the name for each DNS). These files will go to wherever the address in the command prompt is showing.
In case you've never used command prompt, you can change the address command prompt will save the file to if you like using the cd command. "cd .." will take you up a folder level and "cd foldername" will move you in to the folder, if there's a space in the folder name then you need to put quote marks around the folder name.
Edit: To stop this constant ping test cleanly you will need to press the break key on the keyboard whilst the command prompt is selected.