They are sometimes difficult to spot. My company checks log books very carefully, but it appears even then they can get through. We had one pilot who got through the training system who was very full of himself, regaling everyone with tlaes of his experience - he seemed to have done a lot considering he was so young. But his ability didn't match his claims. I wasn't even happy with his taxiing! Eventually, he failed a recurrent sim once too often - and is now flying for one of our competitors. The problem is that although we suspected him of falsifying hours, it's very hard to prove without corroboration from aircraft logbooks etc.
But if someone does come across proof of a fraud, it's beholden of them to report. Ernest Gann in 'Fate is the Hunter' recounts a pilot who in many pilot's opinion was a fraud. When he subsequently crashed an aircraft in another company killing himself and all on board, Gann felt remorse for not 'outing' him at the time. These people are defrauding the majority of honest pilots, and put their passengers at risk. I would have no hesitation in shopping a fake pilot if I had proof.
And sometimes the truth is easy to find. This guy
https://www.express.co.uk/news/uk/51...is-flying-high falsified a RAF career including a fully signed up logbook. His fall came when applying for a command, one of the managers was surprised on checking his RAF logbook to see entries claiming to be on a Squadron that the manager was on at the time. But the manager couldn't remember him! Amazingly, he's back flying. If it were down to me, he would be barred from holding a licence due to a lack of integrity.