I just tried it on a radio button using this JS..
if(document.TravelForm.Sex.checked == false)
{
window.confirm("yes sex");
}else{
window.confirm("no sex");
}
and the html looks like....
<INPUT type="radio" value="M" name="Sex" onChange="hasChanged()"> Male<br>
<INPUT type="radio" value="F" name="Sex" onChange="hasChanged()"> Female