Of course, scientific training frequently is helpful in detecting a hoax, since the scientist may notice errors and inconsistencies not obvious to the untrained eye. The scientist may even have ideas about how he could pull off a better hoax. That doesn't change the fact that scientists are not really trained in either producing or detecting forgeries. On the contrary, if a scientist really trusts the data, the very flaws in the hoax may be taken as indications of behavior that is even more novel and exciting than was previously expected.
This is exactly the mindset that prevails in pathological science: a scientist attempting to perform a real experiment acquires a kind of "target fixation" that blinds him to the importance of serious objections and flaws in the methodology. This happens all the time, even in science that is merely flawed, not pathological. Remember the faster-than-light neutrinos? That would have been a hugely exciting result, and it was not easy to find errors, so it got a lot of attention both from the media and theorists. Scientists tend to be careful in their experiments but then to trust their data, and to expect interesting surprises.
The problem comes when the data comes from an "experiment" performed by someone else. Case in point: crop circles. When the first reports (or, as some would have it, the first modern reports ... yeah, whatever) came out, there were attempts by some scientists to take the phenomenon seriously as a product of nature. Perhaps the most visible of these was Terence Meaden, who proposed some sort of plasma whirlwinds were producing them. As the crop circles got more and more complicated, the proposed means by which they could have been formed naturally became increasingly implausible. By the time it was admitted that many (probably all) crop circles are man-made, these ideas looked very naive.
Another reason why scientists may not be the best at detecting frauds is that the culture of science is very open. It is not enough to say, "I have determined that this is a fraud" -- one must also explain how that determination was made. This, notably, is something that the US Mint refuses to do regarding currency; they give several means that can be used to differentiate legitimate currency from the counterfeit, but they don't disclose all the markers they use. Contrast that with what is said beginning 4:43 in the clip below. "If it was faked, it was done by a human anatomist who was a real genius."
Not necessarily. For one thing, it is not uncommon for artists to study anatomy, but in any event, only the most casual joker would make a foot that was nothing but a bigger-than-average human foot. Perhaps this change was made in artistic inspiration, or perhaps as as a result of anatomical considerations, but in any event there can be no certainty that this was made by an 800-lb biped. What's worse, since this episode was aired in 1977, it has given a clue to anyone planning a sophisticated hoax of a marker that might be looked for. Other markers have been revealed by other sources, so now someone with skill and patience who wants to make fake tracks can prepare a hoax to fool the "experts".
But surely it is too hard for them to fake the footprints? Not at all. Remember, most footprints, whether of Bigfoot or of anything else, are in pretty poor condition. That's an aspect of realism, but it certainly helps anyone planning a prank. More importantly, people are more clever than we give them credit for. Look at the act below. The couple does not claim to have magic powers, and it is pretty easy to guess some of the basics of how the do these things -- but how they are able to do it so smoothly, so quickly, and so often is still amazing. What if someone like this, or like David Copperfield, decided to fake Bigfoot prints? Could he fool scientists who have been trained to trust their data and who have published accounts of what they are looking for? You bet he could!
Some scientists still remember this, and that is why they are not quick to accept the idea that footprints prove the existence of Sasquatch.