Did Michael suspect Fredo before finding out he was the traitor in the family in The Godfather: Part II? Two scenes that made me think of this possibility are when Fredo asks if he knows any one in Havana and Michael says "oh I don't know... Hyman Roth? Johnny Ola?". Also there's the scene when Fredo pretends to meet Ola for the first time and Michael kind of looks on appearing suspicious.
It's my understanding that Michael didn't know Fredo was the traitor until they were at that strip club and Fredo, who previously stated he'd never met Ola, said something to the effect of, "I come here all the time with Johnny".