We don't know.
Senator Geary seems quite au fait with the Corleone's interests however insofar as he's aware of not only the family (which is, perhaps, not surprising) but also their control of various casino / gambling interests
The Corleone family has done very well here in Nevada. You own, or, you control, two major hotels in Vegas -- one in Reno. The licenses were grandfathered in, so there is no problem with the gaming commission.
Now, my sources tell me that -- you plan to make a move against the Tropicalla. They tell me within a week -- you're gonna move Klingman out. That's quite an expansion. However it will leave with one little technical problem. Ahh -- The license will still be in Klingman's name.
So, he has sources..presumably in local/state government.
He's also offering to influence the Licencing Board so being in the know and wielding the power of his office implies a certain level of information at his disposal.
Unfortunately, there is no base novel from which to draw more information as Mario Puzo refused to write one and no novelisation of The Godfather II seems to exist.
The closest we have is The Godfather Returns by Mark Winegardner which covers some of the period involved but Geary/Klingman/the Tropicalla are not mentioned in the book.
Geary is mentioned in Winegardner's follow-on novel The Godfather's Revenge but this also has few clues.