We get the hints about Barzini because he sits at the head of the table, and jumps the gun on the agreement ("then it is agreed--the traffic in drugs will be allowed, and Don Corleone will provide protection in the East") before Vito officially says he'll protect it. (And he's the only Don smoking a cigarette--the others are smoking cigars--showing that Barzini isn't a "moustache Pete.").

I'm surprised that Vito took that long to figure out that it was Barzini all along.