radio button component help, warning if none selected!

[adover]

Hi,
I have had to make a quiz (one of several) which has questions and then 5 radio buttons per group for these questions. These give a scoring which I've assigned to variables and in turn listeners with scores ranging from -2 to 2, what i want is for (when the next button is pressed to move onto the next set of questions) if there is no radio button selected a warning to be produced and no advancement.

I've done this with other similar quizzes before but in thos the scores were from 1-5 so all I had to do was to come up with an if statement checking if the variables value is <1, this one I need to come up with either a null value or use some sort of listener to check if any of the radio buttons in the group have been selected.

Can anyone offer any advice on this? I'm pulling my hair out here!!