How can one distinguish between an ordinary mistake and divine inspiration, especially since sins may appear to be a prelude to future events, as happened with the brothers of Yusuf?

The injustice of Joseph's brothers towards him was not by divine revelation but by the whisperings of their own souls and Satan, for Allah does not command indecency or evil. As for the revelation in the verse (And We inspired to him, "You will surely inform them of this affair of theirs while they perceive not"), it was to Joseph, not to his brothers. The assumption that sins are a path to good is a confusion and a satanic deception; for happiness is attained through faith and righteous deeds, and evil deeds harm their doer. Joseph attained his high status through his excellence and obedience to Allah, not through the disobedience of his brothers. A Muslim must repent from sins that originate from his own self and Satan, and beware of Satan's whisperings.