AutoComplete no search results

I need to change the value of a hidden value, depending if the search of the autocomplete had any results. I know that the ui-autocomplete displays a span that is something like this:

<span role="status" aria-live="polite" class="ui-helper-hidden-accessible">1 result is available, use up and down arrow keys to navigate.</span>

I need to know the results of the search when I am using a onChange event in an input.

My code has to be something like this:

    function onChangeHiddenValue() {
    if(search == 'No results) --> I need to change this
        {
        document.forms['form'].elements['hiddenId'].value = cliente.id;
        }
    else
        {
        document.forms['form'].elements['hiddenId'].value = null;
        }
  }