Yelp, the online review website, has taken on the role of private detective. With more than 71 million visitors monthly, Yelp is using a computer filter to help uncover companies that purchase fake positive reviews, calling out the offenders and then showing the world its evidence. Companies linked to fraudulent reviews are now slapped with an alert:  the Scarlet Letter of the Internet age. “One jewelry store was paying someone $200 [to write a positive review],” said Vince Sollitto, Yelp’s vice president of corporate communications. “It kind of shows that this is an ongoing cat-and-mouse game. People are always going to try to game the system.” Companies, though, say that Yelp reviews can either bring customers flooding into their establishments or running out the front door. “We get good reviews,” said Elham Massarweh, a cafe owner. “That’s how people come in...