I have been an Airbnb host for a while and if you follow your reviews closely, you can tell when someone leaves you low star ratings. In the host dashboard, you only see aggregate stars, but it will break it down as x 5-stars, y-4 stars, etc for each category, so if you know that before your last guest left a review you had 20 5-stars and 3 4-stars and now you have 4 4-stars it's pretty easy to figure out where that came from.