Turnbull I must respectfully dissent.

I believe Michael suspected Fredo's complicity in the hit right from the beginning, but perhaps he didn't want to believe it. He tells Tom he has a working theory but he doesnt let on what it is. He says he must leave "If I think what's happened happened." I think his comment to Tom about Fredo being weak and stupid should be taken in context with his comment that there was a traitor "very close" who "botched" it and who would be "very scared." This all fits Fredo. Tom, probing asks if it could be Neri or Rocco, and Michael merely diverts him with this little speech about everyone being a businessman, and by that thinking anything is possible (ironically it turns out Fredo saw it as "business not personal" because there was something in it for him on his own).

Certainly the question in Havana about knowing Johnny Ola was a trap for Fredo, who by know, Michael probably suspected all the more, if for no other reason,that things in Tahoe were running smoothly.

The scene when they go for a drink in the afternoon is a classic case of "keeping your enemies closer." By now Fredo is racked with guilt, and he has been entrusted to bring the $2 million to the island, which he considers to be an important job. I think he gives it all away to Michael with the very bitter and remorsefule line which said in effect "why didnt we go out like this before?" In other words, "I wouldn't have betrayed you if you had only been closer to me." By this point Michael is certain that Fredo is clueless about his suspicions, and when he shares the information about the impending assassination, he has Fredo drawn back into the fold (Fredo was too weak to stick to his path of betrayal, and Michael was physically too close to allow Fredo to go and talk to Roth's people. In fact the first contact Fredo has with Ola is when they "meet" in the nightclub

Finally, in the famous "Kiss of death scene" Michael says "I know it was you (present tense) you broke my heart (past tense0." So IMHO Fredo's comment at the Superman show was the proof Michael needed that it was Fredo all along.