Yelp has a few ways of knowing if a review is fake. One way is through their automated software that looks for patterns that may indicate a review is fake. This could be things like a new user writing multiple reviews for the same business, reviews that are very short or long, or reviews that use similar language to other reviews that have been flagged as fake.
Another way Yelp knows if a review is fake is through user reports. If enough users report a review as being fake, Yelp will investigate it and may take it down if they determine it is not credible.
Yelp also encourages businesses to report fake reviews written about them. They will look into these reports and take action if they believe the review is not authentic.
So, if you're thinking about writing a fake review on Yelp, beware! You may get caught and it could result in your review being removed.