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.
I think he has already been suspicious. He called Fredo to Havana. He could've asked anyone else loyal to him to bring the money. I don't think any soldier would risk stealing from Michael. He wanted Fredo to come in order to find out whether Fredo knows that sicilian messenger-boy :) Ola or Roth.
Now watch the scene where Michael and Fredo order drinks together. Fredo said before that he has never been to Cuba. However, when ordering drinks Fredo doesn't seem like someone who is first time ordering drinks in Cuba. You can briefly see the cold expression on the face of Michael when he realizes that.
Also when Fredo asked whether there is anyone he knows in Havana, Michael asked specifically about Ola and Roth out of all the people who attended the meeting.
It's just he probably was hoping to be wrong and was struck deeply when his suspicions were finally confirmed.