Stands to reason that it was Clemenza. Think about it - at that point, Sonny was hungry for blood and probably suspicious about everyone. Tessio was the clever one, Clemenza not so.

Now, a generic-looking bullet vest with fish is sent to the Corleone mall. Instantly, Clemenza pipes up that Luca Brasi is dead ... but the speed with which he recognizes Luca Brasi's bullet vest is suspicious. Maybe Luca had name tag on his vest?

Unlikely. Tessio was the clever one, and would realize that the hot-headed Sonny's suspicions would fall upon him if he immediately noted that Luca was dead. He would stay silent and let Clemena relay the news, so that Sonny's suspicions would rest with Clemenza (at this point, Sonny does not altogether trust Clemenza).

In short, it is simply more likely that Clemenza would utter the line.