I think I speculated about that, Oli. Seems to me that since Roth was so tight with Batista, he probably was able to get the customs people to report to him if Michael had brought a substantial amount of cash into the country. That's probably how he knew that "your brother Fredo brought a bagful of money," rather than his "sixth sense." He might also have arranged to have their hotel rooms tossed while they were out. Or, he could have had the hotel management report if either Michael or Fredo had stashed anything in the hotel safe.

I'm guessing that, since Cuba was a gambler's paradise, the country put no restrictions on the amount of cash brought in. But, then as now, US Customs limited Americans to taking out only $10k in cash. I wonder how Fredo got the $2 million out of the US? (He probably told the Customs people, "Coppola and Puzo wrote it into the script.")