  We consider supersymmetric theories where the gravitino is the lightest
superparticle (LSP). Assuming that the long-lived next-to-lightest
superparticle (NSP) is a charged slepton, we investigate two complementary ways
to prove the existence of supergravity in nature. The first is based on the NSP
lifetime which in supergravity depends only on the Planck scale and the NSP and
gravitino masses. With the gravitino mass inferred from kinematics, the
measurement of the NSP lifetime will test an unequivocal prediction of
supergravity. The second way makes use of the 3-body NSP decay. The angular and
energy distributions and the polarizations of the final state photon and lepton
carry the information on the spin of the gravitino and on its couplings to
matter and radiation.
