how to validate textbox having html editor??

how to validate textbox having html editor??

how to validate textbox having html editor??
i am using ckeditor : http://ckeditor.com/download
in my text box to add html editor features?? but on submit i want to check whether the textbox is empty or not??
if text box is empty do not submit to database???
 $(document).ready(function () {

            $("[id*=lblUploadError]").hide();
            // On Click value entered in  textboxes,Works when Add is clicked
            $("#ctl00_InnerContent_Update").click(function () {
                alert("xx");
               // validate ??
            });

        });  
<script type="text/javascript">
     $(function () {
         var config = {
             toolbar:
[
['Styles', 'Format', 'Font', 'FontSize'], ['Bold', 'Italic', '-', 'NumberedList', 'BulletedList', 'Link', 'Unlink', 'Subscript', 'Superscript'],
['UIColor'], ['TextColor'], ['Undo', 'Redo']
]


         };

         $('.jquery_ckeditor').ckeditor(config);
   });

</script>
 <asp:Label ID="lblPageContent" class="lbl" runat="server" Text="Content"></asp:Label>
            <asp:TextBox ID="txbPageContent" class="txtbox jquery_ckeditor" runat="server" MaxLength="2000"
                TextMode="MultiLine"></asp:TextBox>