I have a rule of thumb based on the official star rating of the hotel.
A 4-star hotel should have 4.5 or above average on Google. A 3-star hotel should have 4.2. A 2-star hotel with 4.0 or above can often be pretty good. One of my favourite places to stay in a city that I visit quite often has 4.4* average because although all you get is a bed and a shower and a continental breakfast, those are all great.
For restaurants I try to go for places rated 4.0 or above. If it's trying to be a nice place then I look for 4.4 or so. I once ate at a self-styled "fine dining" place where the 8-course tasting menu was about £100 and it was really poor, and only afterwards did I discover that it was only just above halfway of all restaurants in its city on TripAdvisor. Unsurprisingly it closed soon afterwards.
For car hire, 3.2 or so is actually a decent average, because not many people take the trouble to write a 5* review ("I rented a car, it worked, I brought it back") whereas lots of people think they are getting ripped off because they don't understand how car hire works.
The other thing that amazes me is when people say something like "Very friendly staff. Close to public transport. Small but neat rooms" (this is the entire text of an actual review that I just picked at random) and then give 3. If there's nothing wrong, give 5, people!