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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 this place."

According to various online reports, as many as 30 percent of online reviews are fake, from hotels to toys and books.

Yelp says it's found that some businesses, such as Smith Brothers Appliance Repair, a Long Beach, Calif., moving company, are offering top dollar to have stellar reviews written and posted on their behalf.

"I'm willing to pay you $50 if you can write and post a review for my business," the company posted on Craigslist.

A Yelp employee posing as a writer for hire replied: "I'm interested! Is there some specific text you'd like me to post?"

Smith Brothers did not return ABC News' calls for comment, but the entire exchange is now posted on the company's Yelp review page.

The computer filter that Yelp uses is so top secret the company will not talk about it for fear of giving away too many clues to businesses looking to post phony reviews.

Joe Garvey, the owner of a San Francisco scavenger-hunt business who was busted in a Yelp review sting, said he'd asked someone to write a fake review because "it makes it easier for people to find you."

Garvey told ABC News that he had been dishonest, and that the fake review wasn't worth buying.

"This is not acceptable behavior," Sollitto said. "Frankly, it's not just unethical, but it's probably illegal. And we think we need to let business owners know you can't go out there and try to mislead your consumers."

There are a couple of ways to help spot the real reviews from the fake. Truthful reviews talk about physical space and use specific details like "floor" and "small," while fake reviewers tend to talk about themselves and their companions more than the actual business. Words like "husband" can be red flags.