Radio button validation

I have one group of radio buttons named “group1”. There are 4 radio buttons labeled 1,2,3,4.
What I need is a validatin on pressing submit button.
Something like:

onSubmit = function () {
if (!group1.value){
gotoAndStop(1);
}
else{
gotoAndStop(2);
}

to check if at least one radio button is checked.
But this doesn’t work.