I'd bet it was Bianca that clued him in, by accident. I imagine something like, Bianca sets up shop in town, Marcone sends someone to make a deal with her of some kind, whatever passes for a welcome wagon among organized crime.
Bianca leans on the vampire powers to scare the guy off, expecting that he will just tell his boss that there's no deal and to back off, and that even if he goes to Marcone and goes, "They're goddamn vampires, John," Marcone obviously won't believe him. If she's lucky, Marcone himself will come by to do the job "properly" and she can enthrall him.
Instead, though, Marcone listens to his freaked out lieutenant and decides to look seriously into what he's saying instead.
That's how I imagine it, anyway.