  We study multipartite Bell nonlocality in a framework native of multipartite
Einstein-Podolsky-Rosen (EPR) steering scenarios with a single trusted
measurement device. We derive a closed-form necessary and sufficient criterion
for systems composed of a qubit and N-1 untrusted black-box measurement devices
to violate -- under general dichotomic measurements on the qubit -- a generic
Bell inequality from a broad family of linear inequalities with arbitrarily
many outputs for the N-1 untrusted devices and inputs for all N parties. The
optimal quantum measurements for maximal violation are also obtained. For two
users, and two inputs and two outputs per user, our criterion becomes necessary
and sufficient for Bell nonlocality. Furthermore, in that setting, its form
generalizes recently obtained steering inequalities, which allows us to provide
useful feedback from nonlocality to the detection of steering. Our findings
constitute a practical tool for the study of the interplay between EPR steering
and Bell nonlocality, with potential applications in multipartite information
processing.
