A mouthpiece for a setting device setting a blind rivet element is disclosed which comprises a hollow, which may be a hollow-cylindrical, body having an abutting end and a central through bore through which a pulling mandrel of the setting device is movable. Further, a first endless sealing is arranged adjacent to the central through bore at the abutting end for abutting at a head portion of the blind rivet element. Additionally, a second endless sealing is arranged radially outwardly of the first endless sealing at the abutting end for abutting at a component. Finally, a fluid channel extends at least partly through the hollow, which may be a hollow-cylindrical, body such that an exit opening is provided between the first and the second endless sealing and an entry opening for supplying a testing fluid is provided, which may be remote to the abutting end.