Thanks for the link, TB. I would add to this the fact that Hagen made a comment to Sol to the effect that no matter what else he could do with Sonny and the family there was no way he could stop Luca Brasi. Sol, with his back turned to Tom, in a great bit of acting gets this really smug look on his face and says something like "let me worry about Luca," knowing full well Luca was already sleeping with the fishes. This demonstrates to Sol and probably therefor to Barzini that there was certain secretiveness, therefore a weakness in the Corleone family...how could a real consigliere not be made aware of this? From the look on Sol's face you could see he was already devising his next move.