JavaScript DHTML/Event onMethod/onBeforePrint
"onBeforePrint" Example
<head>
<script language="JavaScript">
function function1() {
window.print();
}
</script>
</head>
<body onbeforeprint="alert("This page is about to be printed")">
<p>Some body Content to print.</p>
<input type="button" value="Print" onclick="function1()">
</body>
</html>