This example demonstrates how to check either jQuery file is loaded or not.
Here’s simple example.
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script src="http://code.jquery.com/jquery-1.6.2.min.js" type="text/javascript"></script>
<script type="text/javascript">
if (jQuery) {
alert('jQuery file is loaded');
} else {
alert('jQuery file not loaded');
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
</div>
</form>
</body>
</html>
Output

Download
jQuery-file-loaded.zip (857.00 bytes)
See live demo