How to detect changes in wp_editor?

How do I detect in JavaScript if the content of a wp_editor tinyMCE instance has changed in WordPress 4.2.2?

I've tried with tinyMCE.editors["content"].isDirty() but the result is always false.

if (tinyMCE.activeEditor.isDirty())
    alert("You must save your contents.");

is working for me.

More: activeEditor, isDirty