This story has been updated.

You might want to think twice about making reservations at that restaurant with only five-star raves. And that hotel with all those one-star pans may very well be fine. The reason? Many of the reviews you’re reading may be fictitious.

It’s difficult to obtain reliable figures on what percentage of online reviews are fake, but research by academic and industry institutions, including the University of Illinois at Chicago and Best SEO Companies, a search engine optimization business, puts the figure at between 20 and 40 percent overall. According to an investigation by Which?, a U.K. consumer advice website, almost half of the five-star reviews for two of the top-10 hotels in Last Vegas were questionable.

Online reviews can make or break a business, and a bustling “review farm” industry has sprung up to write glowing recommendations for pay. You need only type “buy reviews” into Google to have your choice of vendors. Industry efforts to weed them out are ongoing: Amazon has a lawsuit underway against 1,114 Fiverr gig workers who offered to write fake reviews. TripAdvisor published a Review Transparency Report in 2019 which it plans to update biennially. The Federal Trade Commission also brought its first suit against a review farm last year.

A Pew Research Center study found that 40 percent of people “always” or “almost always” check online reviews before making new purchases, while 82 percent do so at least “sometimes.” Consider that against the Harvard Business School study that found that a one-star increase in a Yelp rating leads to a 5 to 9 percent increase in revenue. You can see why restaurants, hotels, bars and attractions are tempted to buy reviews.

Fortunately, there are tools and strategies savvy travelers can use to spot the phonies.

Star ratings aren’t a great indicator “What does a five-star review mean?” said Saoud Khalifah, the founder of Fakespot.com, a tech firm whose software helps identify rigged reviews. “If the food is five-star but the waiter is one-star, how do you score your review?”

There is some value in stars, though. The more reviews there are, the more likely it is that the star rating will have some validity. It’s much harder (and more expensive) to manipulate the star ratings when there are more reviewers. “If there are 10 to 20 reviews, I would be pretty skeptical of the star rating,” said Myles Anderson, the founder of BrightLocal, a marketing firm that helps companies manage their online reputations. “If it’s got 2,000 reviews, it’s much harder to influence,” he said.

Stars can also be used to winnow which reviews you need to read. For example, reviews that award between two and four stars are less likely to be screeds or fakes. A study by professors at Northwestern’s Kellogg School of Management and the MIT Sloan School of Management found that reviewers who provably bought a product were only half as likely to leave a one-star review as unverified buyers.

“Look for the middle of the pack,” said Andy Beal, CEO of Reputation Refinery, a reputation consulting firm. “I tend not to focus on the one- and five-star reviews. Competitors with ill intentions are going to leave one star. If it’s a friend or an affiliate, they are going to leave a five-star.”

If the site breaks out the quantity of ratings behind each star, as Amazon does, look for a pattern. Are there many more threes than fours?

Also look for patterns in the details of reviews. Are there numerous complaints about thin walls and lumpy beds? “There is no smoke without fire,” Anderson said.

Details provide a clue to authenticity Studies by a team at Cornell University found that truthful reviews use words specific to a hotel experience, like “price” and “check-in.” Fake ones use more generic words like “vacation,” “family” and “experience” accompanied by a lot of commas and exclamation marks. Fake reviews are also likely to include more superfluous scene-setting, as in, “My husband surprised me with a lovely anniversary weekend trip . . . ”

You should also be suspicious, Khalifah said, “if reviews are all 100 percent, and they are all one line long — that’s not how anyone writes reviews.”

In fact, uniformly high ratings alone might be a red flag. “We all know nothing is ever picture perfect,” Khalifah said.

Hired reviewers cut corners That gives you an advantage. “They are often pretty lazy about the reviews they write,” Anderson said. Look for “limited text, or identical reviews on two or three sites.” Anderson suggests that you cut and paste a segment of a review into Google it to see if it shows up elsewhere.

Also look up the reviewer to see what else they have written. “If you find someone who has written reviews of 50 different Chicago hotels . . . ” Anderson said. “People don’t stay in that many hotels in one city.”

And so do their employers Look at the time stamp on reviews. “If a business has 200 reviews and 100 came in one month, they were likely purchased,” Anderson said. Businesses tend to buy in bunches, he said. “Then the budget runs out.”

Another hint is if the posts are timed just before heavy travel dates, when hotels and restaurants know people are most likely to be shopping. “We notice a bump in fake reviews right before any travel season,” Khalifah said.

Look closely at photos A photo does a great deal to improve trustworthiness of a post. “Images are really important,” Beal said. “If a review includes images of the property or the food it greatly improves the credibility of the review.”

But the fakers know that, too, and for an extra fee will include photos. One easy-to-spot tell is that bogus reviewers often use a celebrity photo as their avatar. A simple way to double-check the legitimacy of a photo is to do a Google image search, which can reveal whether it was snatched from another site. If there are duplicates, beware.

Take business responses into account Just as important as the comments are the responses from the businesses. “It’s a really big plus for any property that is actually responds,” Beal said. “Then you want to look at what the response is.”

“A lot of businesses come out fighting,” Anderson said. “It’s quite a good rule of thumb to steer clear of the businesses that are pugnacious.” Conversely, he said, no business is perfect, and a conciliatory response “can cover a multitude of things they get wrong.”

Diversify your search It may seem obvious, but don’t do all of your searching on a single site. “My first advice is research as much as possible from as many sources as possible,” Khalifah said. Not just the big travel and booking websites, but vlogs, blogs, Google, Yelp and vacation videos. “That gives you a much better picture.”

A word of caution: Some sites share information, and many are owned by a single company. For instance the Expedia group owns not only Expedia, but also Orbitz, Hotels.com, Trivago and others.

Bring your toolbox The big travel and booking websites count on artificial intelligence to weed out obviously fake reviews. But versions of those tools are available to consumers as well.

Fakespot, Khalifah’s company, offers a Chrome browser extension that analyzes reviews on product and travel sites and awards its own ratings. For instance, on Amazon it can replace stars with its own rating, based on its analysis. Fakespot can also check TripAdvisor and Yelp reviews, although not automatically, and you have to check properties one by one.

Another site, Review Skeptic, which is based on the Cornell research, allows users to cut and paste reviews into a box to be analyzed. They have to be checked one at a time, but the site claims 90 percent accuracy.

Two sites are specific to Amazon, ReviewMeta and Review Summarizer. ReviewMeta runs 12 tests for veracity, while Review Summarizer compares specific attributes of a product and calculates the percentage of positive and negative reviews for that attribute.

The Airbnb conundrum Reviews on the vacation rental site present a special problem. As with eBay and Uber, the guest rates the service, but the service also rates the guest. If a guest writes a critical review, they might get one in return, narrowing the choice of Airbnbs that are likely to accept them as customers. You can try to parse reviews for a hint of dissatisfaction, but there is no effective strategy for spotting fakes. “We receive a lot of requests to add Airbnb and may support it in the future,” Fakespot’s Khalifah said.

In the meantime, you’re on your own.

Furchgott is a writer based in Baltimore. His website is furchgott.com. Find him on Twitter: @royfurchgott.