  Opto- and electromechanical systems offer an effective platform to test
quantum theory and its predictions at macroscopic scales. To date, all
experiments presuppose the validity of quantum mechanics, but could in
principle be described by a hypothetical local statistical theory. Here we
suggest a Bell test using the electromechanical Einstein-Podolski-Rosen
entangled state recently generated by Palomaki et al., which would rule out any
local and realistic explanation of the measured data without assuming the
validity of quantum mechanics at macroscopic scales. It additionally provides a
device-independent way to verify electromechanical entanglement. The parameter
regime required for our scheme has been demonstrated or is within reach of
current experiments.
