Check it out online first: it’s a like a mantra for consumers, the sort of sensible-sounding advice that crops up in personal-finance columns all the time.

And there can’t be many people who haven’t checked out online reviews before buying something pricey or booking a hotel. Personal endorsement has always been the most trusted form of advice.

But last week the UK competition and markets authority (CMA) launched an investigation into a “number of companies” that may have paid for those reviews. In February the watchdog issued an open call for consumers’ experience and heard stories of fake reviews being posted on review sites; negative reviews not being published; and businesses paying for endorsements in blogs and other online articles without this being made clear to consumers.

On foot of the complaints, it says, it is using its consumer enforcement power to investigate a number of as yet unnamed companies.

Fake reviews aren’t new but they are big business. Type “fake reviews” into your search engine and a world of job opportunities as a fake reviewer opens up – with many sites promising a few dollars in return for a review.

A good review is good for business. The CMA estimates that just over half of UK adults use online reviews before making a purchase and there’s no reason to think that that figure isn’t the same or even higher here. Last week’s report said £23 billion (€32 billion) a year of UK consumer spending is potentially influenced by online reviews, so it’s a huge consumer issue.

As part of its plans for assuming the presidency of the International Consumer Protection and Enforcement Network on July 1st , the CMA is proposing a project on online reviews and endorsements.

In 2013, New York state attorney general Eric Schneiderman called fake online reviews “the 21st century’s version of false advertising”. He was speaking after Operation Clean Turf, a year-long sting by his office, caught and fined (to the collective tune of $350,000) 19 companies that had posted fake reviews online.

Teams to weed out fake reviews

Websites where the integrity of the reviews are key to the business model, such as TripAdvisor, Airbnb and Yelp, have dedicated teams to weed out fake reviews. In Yelp’s case it is thought about 10 per cent of its employees are dedicated to the task. Last December, Italy’s version of the CMA fined TripAdvisor €500,000 saying it should stop “publishing misleading information about the sources of its reviews”.

But rooting out the fakes isn’t always easy. What’s to stop a business owner assuming a false identity – it’s not exactly rocket science or an unusual thing to do online – and posting glowing reviews of his or her own business. Delving deep into algorithms and IP addresses will uncover the fakers eventually but still, for a consumer idly roaming the web for bit of advice about a purchase, the first reaction to most reviews is one of trust.

And how do you know those glowing reviews for that interesting piece of kit you want to buy were all the manufacturer received from previous buyers – and that they didn’t remove any negative one?

Astroturfing

The business of online fake reviews has a name: astroturfing; and it has been around long enough for universities to devote academic time to it. A study by the University of Illinois found 30 per cent of online review across goods and service categories were most likely bogus.

Cornell University suggested ways consumers could spot fakes. For example, look out for excessive use of the personal pronoun. If every sentence begins with "I", it could be faker trying their $5-a-review best to convince you that they were in fact in that restaurant or used that taxi service. Of course, they could be just your average internet egotist – it's hard to say.

Look out too, Cornell said, for wildly superlative language, though, let’s face it, you have to be pretty keen about a product before you could be bothered to log on and give it a positive review. For a consumer, it’s complicated.

Cornell came up with handy interactive software, the review skeptic which it said could identify fake hotel reviews with nearly 90 per cent accuracy. You just type the review into the onscreen box and let the technology sniff out the spoofs.

Whatever the result of the CMA’s investigation, just highlighting the problem of fake reviews and reminding us trusting browsers to beware is a consumer service in itself. Twitter @berniceharrison