​x
 
<html>
<body>
​
<form id="form1">
<input type="radio" id="radio1" />
</form>
​
<p>The form containing the radio button is: 
<script type="text/javascript">
x=document.getElementById('radio1');
document.write(x.form.id);
</script></p>
​
</body>
</html>
​