They suspected Clemenza because Clemenza was the one who brought Paulie into the Corleone family. On the DVD bonus disc there is a deleted scene explaining this whole scenario regarding Sonny suspecting Clemenza, but they soon find out that Clemenza had nothing to do with Paulie setting up the old man. It's a really informative scene and as I've said many many times before it should have been left in the movie.

Don Cardi