potresti fare così
codice:
<script language="JavaScript" type="text/JavaScript">
function calcola(cc){
var ff = cc.form
for(var i=0;i<ff.elements.length;i++)
if(ff.elements[i]==cc)
alert(i)
}
</script>
<form name="NomeForm">
<input type="checkbox" name="opt[]" value="1" onclick="calcola(this);">
<input type="checkbox" name="opt[]" value="2" onclick="calcola(this);">
<input type="checkbox" name="opt[]" value="3" onclick="calcola(this);">
<input type="checkbox" name="opt[]" value="4" onclick="calcola(this);">
<input type="checkbox" name="opt[]" value="5" onclick="calcola(this);">
<input type="checkbox" name="opt[]" value="6" onclick="calcola(this);">
</form>