Some people here think Michael suspected Fredo because of the searching looks he gives Fredo at two points: when Fredo asks if there's anyone in Havana that he knows; and at the nightclub when he says to Ola, "I don't think you've met my brother." Could be. But then we have to take into account that Michael asked Fredo, "Can I trust you with something?" and then told him that Roth would never see the New Year. And, Michael's absolutely devastated when Fredo makes his fatal gaffe at the Superman show.

I believe that Michael was telling the truth when he told Tom, after the shooting: "Right now, you're the only one I can completely trust [emphasis added]...Fredo? He's got a good heart, but he's weak and stupid, and this is life and death." I think he meant that he didn't suspect Fredo of deliberately betraying him ("good heart") but that Fredo might have inadvertenly contributed to the shooting through his own stupidity. So, when Fredo made his fatal gaffe, Michael was devastated to learn that Fredo was consciously conspiring with Roth and Ola.