  We report on the first experimental reconstruction of an entanglement
quasiprobability. In contrast to related techniques, the negativities in our
distributions are a necessary and sufficient identifier of separability and
entanglement and enable a full characterization of the quantum state. A
reconstruction algorithm is developed, a polarization Bell state is prepared,
and its entanglement is certified based on the reconstructed entanglement
quasiprobabilities, with a high significance and without correcting for
imperfections.
