[Javascript] How do you cycle thru the radio buttons on a web form ?

[dev at qroute.net]

I am running a survey.
I need to detect the radio optioned questions in it which are being
submitted with no option selected on them.
I know by default and by design principles, radio's gotto be presented with
an option already pre-selected, but in my particular case, I want to send
the form with nothing pre-selected and force the user to select one.

At submit time, I do not want to submit the form if a radio question is left
blank.
How do you accomplish it ?