how do I get the reponse text from ajax / jquery?

Use on your client side ajax like this

 $.ajax({
        type: "POST",
        url: "insert-data.php",
        data: 
    {student_name:student_name,student_roll_no:student_roll_no
     ,student_class:student_class},
        dataType: "JSON",
        success: function(data) {
         $("#message").html(data);
        $("p").addClass("alert alert-success");
        },
        error: function(err) {
        alert(err);
        }
    });

in server side after query excecute you may use it give success when you query success false when your query has fault

if($stmt->execute())
 {
$res="Data Inserted Successfully:";
 echo json_encode($res);
}
 else {
 $error="Not Inserted,Some Probelm occur.";
echo json_encode($error);
  }

Looks like somehow your jQuery is returning the XMLHttpRequest object, instead of your response.

If that is the case, you should ask for its responseText property, like this:

 $.ajax({
    type: 'POST',
    url: '/ajax/watch.php',
    data: {'watch':'aukcia', 'id':aukciaID},
    complete: function(r){
       alert(r.responseText);
    }
 });

However, if that does not work, you might be actually receiving a JSON response, and the [object Object] you are seeing might be your browser's representation of your JSON response.

You should be able to inspect its contents by navigating around the object properties. However, if you want, you can also tell jQuery not to parse your JSON response, by including dataType: 'text' on your call:

 $.ajax({
    type: 'POST',
    url: '/ajax/watch.php',
    data: {'watch':'aukcia', 'id':aukciaID},
    dataType: 'text',
    complete: function(data){
       alert(data);
    }
 });

For more information, see: http://api.jquery.com/jQuery.ajax/