  In this Letter, we investigate the environmental dependence of dark matter
halos in theories that attempt to explain the accelerated expansion of the
Universe by modifying general relativity (GR). Using high-resolution N-body
simulations in f(R) gravity models which recover GR in dense environments by
virtue of the chameleon mechanism, we find a strong environmentally-dependent
difference between the lensing mass and dynamical mass estimates of dark matter
halos. This environmental dependence of the halo properties can be used as a
smoking gun to test GR observationally.
