Until this day I thought that Fredo and Roth had direct or indirect connections in Havana before the New Year's Eve party but Roth was too smart for that because he would have known Fredo would have been too stupid to pull off any secret contact with him
because Michael was right there to keep an eye on him (BTW Michael knew this too which may explain the long standing question -- "Why would Michael tell Fredo he planned to assassinate Roth?).

No Roth probably knew Fredo's flight number, and his suveillance had him getting out of the car at the hotel and clutching to the briefcase handcuffed to his wrist.

The evidence? When Fredo "met" Johnny Ola at the party, Fredo looked stricken with terror...he didn't know how to handle the introduction. It was Ola who bluffed through it, and Fredo followed his lead.

Thus it was Roth, with the help of the Cubans that was really his "sixth sense."