How do you know that God does not want you to succeed in a certain matter because it is evil and contains no good?

The servant must entrust his affairs to Allah and rely upon Him, and seek counsel and divine guidance (Istikhara). If, after that, the matter does not become easy for him, then let him know that there is no good in it, and that Allah has diverted it from him for a wisdom, and that the good lies elsewhere.