You'll need to access the group (name attribute) and then look for the values:
<form name="frm1">
<input type="radio" name="sex" value="male" checked="checked" /> Male<br />
<input type="radio" name="sex" value="female" /> Female
</form>
var sex1 = document.getElementsByName('sex');
var sex2 = document.forms["frm1"]["sex"];
if (!sex1[0].checked && !sex1[1].checked)
{
alert('Nothing selected.');
}
As far as I know there is no selectedindex, so you'll need to loop through the collection.