Villefort uses his super-stealth ways to find out more about the Count.
He discovers that the Count has two old buddies living in Paris: Lord Wilmore and Abbé Busoni.
Villefort gets the police to go visit Abbé Busoni and get the real dirt. Abbé Busoni (who is really the Count in disguise) speaks highly of the Count of Monte Cristo, having known him for years (wink, wink), and he tells them that the Count has one enemy in the world: Lord Wilmore.
Villefort goes to see Lord Wilmore (who is really the Count in disguise).
Wilmore tells Villefort that the Count made all of his money off of a silver mine which he discovered somewhere in the Middle East. He tells Villefort that the Count has bought the house in Auteuil because he hopes to find a mineral spring.