In his last face-to-face with Michael in Havana, Roth says: "My sixth sense tells me that your brother Fredo brought a bag full of money. Where is it?"
How did Roth know that Fredo brought the money? He might have inferred that Michael would have brought Fredo to Havana for only that purpose. His pals in the Cuban government might have tipped him (although if they knew he had $2 million in that suitcase they would have grabbed it).
Or, Fredo might have told Roth.
Your view?
In those days booking flights was a big deal. Flights from Tahoe to Havana could have been easily monitored. Most likely Roth either paid off or tapped the phone of the Corleone's travel agent. All he really needed was someone at Havana airport confirming that Fredo and the suitcase had landed and cleared Batista's customs.
