There should be an event message when the config is out of sync. I can't seem to locate any documentation on this. But if you have syslog setup you might be able to find the message in your log history and create an alert on this.
You probably should go through this kb on testing config sync to be sure your don't have a current issue.
http://kb.juniper.net/InfoCenter/index?page=content&id=KB9817