  We present a device-independent protocol to test if a given black-box
measurement device is entangled, that is, has entangled eigenstates. Our scheme
involves three parties and is inspired by entanglement swapping; the test uses
the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality, checked between each
pair of parties. Also, focusing on the case where all particles are qubits, we
characterize quantitatively the deviation of the measurement device from a
perfect Bell state measurement.
