Online reviews can make or break a local business, which is why the incentive to cheat with paid-for, malicious or fraudulent reviews is so great. Weeding out fraud is an inexact science; sites such as TripAdvisor, Google+ Local and Yelp use a mix of algorithms and human moderators to separate fraudulent reviews from genuine.

Of all the major review sites casting a dragnet, Yelp stands out for its aggressiveness in trying to catch fraudsters: 20% of reviews written and submitted are never displayed. "Some legitimate reviews may be caught up in this," said spokesman Vince Sollitto. "It's a continuing cat-and-mouse game that will always be with us."

It's about to get even tougher for repeat offenders. The site is considering naming and shaming the businesses who continue to deploy fraudulent reviews. "We are thinking of ways to alert consumers of more concerted efforts by business to mislead them," Mr. Sollitto.

How many online reviews are fraudulent? No one knows for sure, and the estimates are all over the map. A Gartner study in September predicted that the amount of fake reviews would grow to 10% to 15% by 2014 and that "at least two" Fortune 500 brands would face litigation from the Federal Trade Commission for the practice in coming years. "We think more reviews are going to be faked because of the high trust in them, and not everyone is going to be caught," said Gartner analyst Jenny Sussin.

That prediction includes fake "likes" and followers in social media. Bazaarvoice, which powers reviews for Macy's and 1-800-Pet-Meds and many others, says its fraud rate was less than 1% over the past four months, in part because 85% of its comments are driven by a post-transaction email, meaning the commenter really did buy something or stay in a hotel room.

One thing is certain: In a world increasingly filled with online commentary, likes and reviews, law enforcement is overmatched. The law in the U.S. is that if you've been paid to endorse a product or service, you must disclose it. The FTC fined Legacy Learning Systems $250,000 in 2011 for paying fake reviewers based on sales they generated.

Given the stakes, a cottage industry has sprung up to help businesses keep track of their reviews and identify fraud when it happens. Reputation management companies like Reputation.com have added tools for comment monitoring, and a company more specifically aimed at detecting review fraud, Review Trackers, launched earlier this year.

Review Trackers is working on its own technology, but founder Chris Campbell said it will never replace the humans who now read every single new review mentioning its clients. "It is difficult to identify a fake review without the human element," he said.

FALSE POSITIVES

When it comes to detecting fakes, humans and algorithms make mistakes. And while all the big review sites have a process that allows businesses to flag reviews they believe to be fraudulent, paid for, maliciously placed by a competitor or unfair, they generally don't have one to challenge legitimate reviews that are mistakenly flagged as comment spam.

After Google removed hundreds of rave reviews for three car dealerships in Wichita, Kan.; Boulder, Colo.; and Canton, Mass.; from its Google+ Local pages, the dealers filed complaints with the FTC. "Google believes it can do whatever it wants and has no accountability," dealer Scott Pittman at Suzuki of Wichita told Automotive News.

"We acknowledge in trying to strike balance in removing spam reviews and keeping legitimate reviews, there will be some error," a Google spokeswoman said. "Illegitimate reviews are a problem across the internet; it's something we are constantly working on to better detect and handle the abuse."