The only other point that Fredo could have tipped anyone off was immediately after the scene where Michael predicts his assassination (Roth's rather racist comments about the local doctor). Aside from "getting a real" drink, it's the only scene where Michael and Fredo are not together until the New Year.

Just hypotheticals here -- and assuming Batista had not resigned -- Michael might have outmaneuvered Roth by traveling with the Senators. In the Batista regime, the US Ambassador was at least the second most powerful man on the island. To have an assassination in full view of several powerful US government officials would have been a disaster.