If jQuery is installed and running : jQuery « jQuery « JavaScript Tutorial

<html>
  <head>
    <script type='text/javascript' src='js/jquery-1.3.2.js'></script>
    <script type='text/javascript'>
if ($) {
  $(document).ready(
    function() {
      $('p').addClass('Loaded');
      $('p').text('jQuery loaded and running!');
    }
  );
}    
    </script>
    <style type='text/css'>
p.Loaded {
    color: green;
    border: 1px solid green;
}    
    </style>
  </head>
  <body>
    <p>
        jQuery is not loaded.
    </p>
  </body>
</html>

30.1.jQuery
	30.1.1.	Adds two functions into the jQuery namespace.
	30.1.2.	Adds two plugin methods.
	30.1.3.	Is jQuery ready
	30.1.4.	Create Text node from query list
	30.1.5.	Show the order in the jQuery object.
	30.1.6.	Pass array returned from jQuery to a function
	30.1.7.	If jQuery is installed and running