Michael started to slowly test Fredo in Havana when Fredo brought the money, when they were out having banana daqaris, and at the NYE reception. But I would add these were more to gauge Fredo's reactions rather than deep suspicions.
Goombah, you provide a nice, logical bridge between Michael's "good heart" speech at Tahoe and the suspicious looks in Havana. But then, another piece of illogic comes along to confound us:
If Michael was at all suspicious, or even unsure, of Fredo, why did he tell Fredo about his plan to whack Roth, and when he was going to do it? And, even if he wasn't at all suspicious or unsure of Fredo, why tell him at all, since he didn't want Fredo to do anything to help him? Since Fredo was "weak and stupid," as Michael told Tom, there was a real chance he could screw things up even unintentionally.
