Before dining, shopping, or checking into a hotel, millions of Americans turn to online sites like TripAdvisor, Zagat, and Yelp to read peer reviews of businesses, but many of those reviews are turning out to be fake, and sites like Yelp are cracking down on companies abusing the system.

“Some businesses will go to extreme lengths to bolster their reviews,” said Eric Singley, vice president of Yelp’s consumer products and mobile division, in a statement.

To combat fraud on its site, the Yelp security team conducted an undercover sting operation on websites like Craigslist, where they found some businesses posting employment ads, offering to pay $10 for each bogus positive review.

Yelp workers went onto Craigslist to sell positive reviews and some businesses responded, paying for good reviews, so Yelp is now posting consumer alerts on the pages of companies suspected of soliciting and using reviews-for-hire.

The consumer alerts will stay up for at least 90 days, and even longer if the suspicious activity continues. Yelp will also inform visitors when a business has a large number of reviews posted from the same computer, a red flag for fake reviews.

According to a Nielsen survey released earlier this year, 70 percent of global consumers trust online consumer reviews.

Yelp averages approximately 78 million visitors each month offering “strong incentive for some businesses to try to game the system,” the company said.