jQuery，AJAX - 如何判断脚本是否返回false

I'm using jQuery and AJAX to validate my form when someone creates a new user on my website. I'm programming in OOP PHP, together with the jQuery and AJAX. I'm using this code:

$.ajax({  
        type: "POST",  
        url: "includes/classes/handler.php?do=addLogin",
        data: dataString,  
        success: function() {
            $('.sideBarNewUserWrap').fadeOut();
        }
    });  
    return false;

But how do I return an error message, if the e-mail already exists?

Hope it's info enough, else I'll just add some more. Thanks in forward :)

* UPDATE *

This is my PHP checking if email exists:

$email_count = mysql_num_rows($check_email);
    if($email_count){
        return false;
    }

* UPDATE *

      success: function(data){

            if(data.error){
                $('.sideBarNewUserWrap').fadeOut();
            } else {
                $('.sideBarNewUserError-email').fadeIn();
            }

Now this looks pretty much as a failure because. if(data.error) then it's okay?

Shouldn't it be something like:

if(date.error){
  //Error message
}

And not the other way around?

Well, If I try to enter an email which already exists, it tells me as it should, but why does this work? In my eyes I'm doing something wrong here?

You can get the response in the function:

$.ajax({  
        type: "POST",  
        url: "includes/classes/handler.php?do=addLogin",
        data: dataString,  
        success: function(response) {
            if (response == "ok")
            {
                 $('.sideBarNewUserWrap').fadeOut();
            }

            else
            {
                // error happend
            }
        }
    });  
    return false;

You can return string, int in PHP or even XML, JSON, whatever you want to validate on client side

basically in the handler.php you should verify whether email already exists or not and then send to the client (at least) two different responses (eg. 0: email exists, 1:ok).

In the success callback you can read the response data so you can tell the user the operation status

php:

$result = array('error' => true); // or $result = array('error' => false);
echo json_encode($result);

js:

success: function(response) {
    if (response.error) {
        // ...
    }
}

You can return data by using an echo in your handler.php file. To receive this in jQuery just place a parameter in the success function of the Ajax function.

success: function(returnedValue)
{
// Here you check if returned value e.g. "returnedValue == 1"
}