sotik,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<script>
function change(checkbox){
var a = document.querySelectorAll("input[type=checkbox]");
for (var i=0; i<a.length; i++) {if(a[i] == checkbox) return i}
return -1
}
</script>
<input type="checkbox" onclick="alert(change(this))"/>диезов<br>
<input type="checkbox" onclick="alert(change(this))"/>бемолей
</body>
</html>