It was recognized by scientist right away to be a fake.
“Almost from the outset, Woodward’s reconstruction of the Piltdown fragments was strongly challenged. At the Royal College of Surgeons copies of the same fragments used by the British Museum in their reconstruction were used to produce an entirely different model, one that in brain size and other features resembled modern man. Despite these differences however, it does not appear that the possibility of outright forgery arose in connection with the skull.
Approximately 1915, French paleontologist Marcellin Boule concluded the jaw was from an ape. Similarly, American zoologist Gerrit Smith Miller concluded Piltdown’s jaw came from a fossil ape.
In 1923, Franz Weidenreich examined the remains and correctly reported that they consisted of a modern human cranium and an orangutan jaw with filed-down teeth. Weidenreich, being an anatomist, had easily exposed the hoax for what it was. However, it took thirty years for the scientific community to concede that Weidenreich was correct.
In 1915, Dawson claimed to have found fragments of a second skull (Piltdown II) at a site about two miles away from the original finds. So far as is known the site has never been identified and the finds appear to be entirely undocumented. Woodward does not appear ever to have visited the site.