...in a very early scene at Anthony's First Communion reception, when Fredo "apologizes" to Michael about his wife. Notice the expression on Fredo's face when Michael tells him he don'y have to apologize. The expression (on Fredo)is one of "knowing something", "worry"' and almost "guilt" simutaniously. Is that a clue? ...
Since we know that by the time of Anthony's Communion Fredo had already been working with Ola & Roth for god knows how long, then yes he would have a look of guilt and knowing something on his face.
However, that look does not necessarily convey that he was aware of and taking a crucial role in a planned assasination of Michael that was to take place that very evening.
