I took a look at the CKEditor API and found this useful event:
http://docs.ckeditor.com/#!/api/CKEDITOR.editor-event-afterCommandExec
It's not working for anything that requires a dropdown ( classname, font, fontsize ) but it works for any buttons like bold, link, undo.
Does anyone have any ideas why this doesn't work or if there is some other event for these kind of toolbar plugins?