Wow,this is a first,I'm the only one who thinks he did,if not know,that have a strong suspicion. Despite him being incredably dull,he had to have picked up some things from Vito. Like I said before Al is an important guy,much too important to be a boat chauffer,and if Fredo can tie his shoes,he would have noticed this.If Michael all the sudden appointed him "in charge of the boats" Fredo would have had to notice something wasnt right.Even if he wasnt 100% sure,"Al's going to kill me right now" he wasnt totally ignorant to the fact either.