Check if jquery library exist
<p>How to check if jQuery library exist in head tags?</p>

<pre><code>&lt;head&gt;
&lt;script type="javascript/text" src="http://code.jquery.com/jquery-1.6.2.js"&gt;&lt;/script&gt;
&lt;/head&gt;
</code></pre>

<p>and if not exist how do I load in head tags the jquery library, I'm doing a plugins and I want to load my plugins script in jQuery and also do able to check if jQuery library exist so my jQuery script will run</p>
