The trick which hasn't failed me yet is to look at exclusively 1-star reviews. I've been using Google Maps, and I'm assuming this trick will work with other services also. First, look at the ratio of 1-star to other reviews - do the 1-star reviews create a "hockey stick"? What's the ratio of 1-star to 2-star? 1-star to all reviews?
Second, see what people are complaining about, for example, these 2 1-star reviews reveal vastly different experiences:
Cafe: Horrible staff and horrible food. The owners wife argues with the customers, over charges and makes sure to let you know shes overworked and exhausted. We paid over a 100 dollars on lunch for 4 people. One of those orders was on a childs menu. The resturant is very neat looking and has the potential to be successful. They need a new cook and new staff. Its clear everyone hates their jobs.
Pub: We picked this place based on the high reviews. I specifically wanted to dine here for the crab cakes. I'm fairly certain there wasn't crab in them since they where mostly batter. The flavor was lacking and the aoli was gross. I didn't say anything and should have. I just left and I'm still hungry. Price is extremely high too. Needs new menu.
These restaurants are within 5 miles of each other. As the second review was entirely about the food, it's safe to say there were no service, billing, parking, or other issues - especially since other reviews back this up. And food ratings frequently reflect preference - maybe I would enjoy the crab cakes. So the second establishment would get my business.
But yes, there's no source quite like the opinion of a local, directly from their mouth. Even better than the above system is to find a bar/pub/coffee shop/etc where the locals go, and ask them how things are going in the area.