We consider stable modified theories of gravity that reproduce galactic rotation curves and the observed amount of weak lensing without dark matter. In any such model gravity waves follow a different geodesic from that of other massless particles. For a specific class of models which we call 'dark matter emulators', over cosmological distances this results in an easily detectable difference between the arrival times of the pulse of gravity waves from some cosmic event and those of photons or neutrinos. For a repeat of SN 1987a (which took place in the large magellanic cloud) the time lag is in the range of days. For the recent gamma ray burst, GRB 070201 (which seems to have taken place on the edge of the Andromeda galaxy) the time lag would be in the range of about 2 years.