Not too difficult to fake being an airline pilot these days if you have some flight training covering the rudimentaries of airline flying. With the advent of flight sim and VR software plus home based sim hardware like flight controls connected to the computer, armchair pilots can actually go quite far in mimicking actual airline operations. They will only be found out as fakes when met with actual non normal situations in actual flights or if someone bother to do extensive background checks.
On normal flight operations there will certainly be little to tell them apart from actual pilots if they are also good actors, and many got away easily. The danger is that if the do get involve in a real fatal accident, and if the insurers do find out they are fakes then what are the passengers' and other crews rights with regards to insurance claims?