If I pay attention to the word "KNOW" as yelled out at us in the thread title, I would have to say that it is very unlikely that we actually "KNOW" what cases are hoaxes or frauds because all we've got to go on is second and third-hand information. We weren't there to witness either the hoaxing or the real event. We're just parroting articles and videos and whatever so and so says, and making judgement calls based on what we think is most likely to be true. But is that really the same as knowing? I don't think so.
For example, I would submit that only Travis Walton really knows if his story is a hoax or not. Even the Billy Meier case with those seeming obvious photo fakes might not be what we think. I've seen it proposed that the obvious fakes aren't even photos taken by Meier or advocated by him as genuine. IMO the only time we can say with some certainty we really know that some experiential claim is true, is when we experience and investigate it ourselves. But even then, it's possible that we might be misinterpreting the evidence perceived through our senses and/or misinterpreting data gathered from technology, or perhaps we're not misinterpreting the data but the data itself is in error due to undetected malfunctions, and we're drawing inaccurate conclusions, even though our own thinking and experience is just fine. In the end it begs the question: How do we really know anything experiential is really what we think it is?