technically you still can't distinguish acceleration from gravity they are in fact one in the same the simple direction difference between the negative and positive masses accelerations are not qualifiers for discrimination there is no way in a single reference frame to distinguish the negative and positive masses i e whether the earth is negative or positive mass and the object in question is positive or negative further the magnitude of the acceleration should be identical for each object i would suggest reading up on hawking's idea in his universe in a nutshell book that mass energy gravitational energy in essence you can create all the matter you want as long as it has gravitational pull which he regards as a negative energy if this is the case then there could in fact be negative mass with positive gravitational energy which behaves in an opposite manner i would presume that annihilation due to matter antimatter interactions would have gravitational energy considerations as well but in any case the mere fact that negative mass would accelerate in a different direction in a gravitational field does not in itself preclude its existence especially since gravitational laws were formulated without accounting for this eventuality