Yes, I beleive in the book that they actually had their connections in the phone company give Sonny a list of every phone call both Clemenza and Paulie made over the past few months so they figure out which was the traitor.
"Somebody told me when the bomb hits, everybody in a two mile radius will be instantly sublimated, but if you lay face down on the ground for some time, avoiding the residual ripples of heat, you might survive, permanently fucked up and twisted like you're always underwater refracted. But if you do go gas, there's nothing you can do if the air that was once you is mingled and mashed with the kicked up molecules of the enemy's former body. Big-kid-tested, motherf--ker approved."
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