I've said it before, I think he was already suspecting Fredo very early on, but he couldn't be 100% sure until Fredo would comfirm his suspisions.

But in order to reveal himself, Michael needed Fredo to absolutely trust him first. That's why he shared his plans with him when they are at the terrace in Havana. It was all part of his game. Michael's facial expressions pretty much reveal everything in these scenes.

I think it was more important for Michael to know who the traitor was, than to keep his plan to kill Roth a secret. He was already taking risks to be in Cuba in the first place.