detect ajax request by jquery
<p>I'm working with asp controls and I have some dropdownlist which in selection go back to the server and do some action ( via asp ajax )
what I'm l looking for is detecting via jquery when the ajax call is starting 
I've tried the following call :</p>

<pre><code>  $.ajaxSetup({
                    beforeSend: function (jqXHR, settings) {
                        alert("ok");
                        return false;
                    }
                });
</code></pre>

<p>and </p>

<pre><code>also  $(document).ajaxStart(function () {
                   alert("OK");
                });
</code></pre>

<p>but none of this worked</p>