People love to bash yelp, but it's merely a reflection of the fact that People Are Dicks

Check the reviews, there's also how many reviews and friends the person has. The lower these two numbers are, the most likely the review is fake (either pro or against)

Also check how balanced the review is. Most legitimate ones are fair, and won't go over the top either way

And of course, there's always someone that's going to complain about a sushi restaurant serving raw fish.