I thought that mormons had prophets that could figure these things out from God.
Church leaders don't directly involve themselves with such matters. Although church historians were deceived by Hofmann, in the case of the RLDS historian, he saw to it that an document of great interest to the RLDS was studied by a document expert and a handwriting expert, since the historian was not himself skilled in these areas. The forgeries were so skillfully done, using paper and ink from the period, that the experts reported to the historian that the document was genuine. Hofmann's forgery had even used phrases that Joseph Smith was using about that time. The historian reported what the experts had told him, and the church leaders, leaning on those reports, decided to go with it.
We all make mistakes of varying kinds. Hopefully, we can be wise enough to avoid making the same mistakes again.
"And now if there be fault, it be the mistake of men; wherefore, condemn not the things of God, that ye may be found spotless at the judgment seat of Christ."
Then what good is it having 'prophets', since they involved themselves in it.