My assumption watching the movie was just that Don Vito simply had the instinct to know which was beyond what a normal person would figure out.
I also want one or two of those lupuaro (sp) shotguns with slings so I could walk around town with them slung over my shoulder. Also with a bandoleer of shotgun shells.
They really look neat and deadly. I suspect that really is what a Sicilian bodyguard would use.