I stayed at an awful B&B years ago and sometimes look at new reviews of the place as I'm intrigued. There are several bad reviews, all saying similar things to me.
Recently, two 5-star reviews have appeared on TripAdvisor and I'm suspicious. They just seem a bit overly-glowing.
I do read reviews online before booking somewhere, and if a place has a few bad reviews among lots of good ones then I often tell if the reviewer was being unreasonable. But false positives can be harder to spot.
One red flag for me is when the staff are all referred to by name, especially at a restaurant where apparently the reviewer learned the names of all the staff. So "Jenny was a great host. Bob, our waiter, listened attentively to our requests and Mike at the bar is great at cocktails" etc.