How Did Vito Corleone know Don Fanucci Was A Fraud?

In the godfather part 2, Don Fanucci is the feared mafia boss in vito's neighborhood, little italy. He goes around collecting his pay from companies and stores in exchange for protection. But this isn't the story. there is some dark secrets fanucci holds that no one is aware of until vito comes along and does. #thegodfather #vitocorleone #michaelcorleone #robertdeniro