<script>
function controlla(nomeform) {
var myform = document.forms[nomeform].elements;
for(var i=0; i<myform.NOMERADIO.length; i++) {
if(myform.NOMERADIO[i].checked) return true;
}
alert("nessun radio selezionato");
return false;
}
</script>

<form name="nomequalsiasi" onsubmit="return controlla(this.name)">
<input type="radio" name="NOMERADIO" value=1>
<input type="radio" name="NOMERADIO" value=2>
<input type="radio" name="NOMERADIO" value=3>
<input type="radio" name="NOMERADIO" value=4>
<input type="submit" value="invia">
</form>