  The standard cosmological model is based on general relativity and includes
dark matter and dark energy. An important prediction of this model is a fixed
relationship between the gravitational potentials responsible for gravitational
lensing and the matter overdensity. Alternative theories of gravity often make
different predictions for this relationship. We propose a set of measurements
which can test the lensing/matter relationship, thereby distinguishing between
dark energy/matter models and models in which gravity differs from general
relativity. Planned optical, infrared and radio galaxy and lensing surveys will
be able to measure $E_G$, an observational quantity whose expectation value is
equal to the ratio of the Laplacian of the Newtonian potentials to the peculiar
velocity divergence, to percent accuracy. We show that this will easily
separate alternatives such as $\Lambda$CDM, DGP, TeVeS and $f(R)$ gravity.
