If everything is moving at the same rate (inertial, thus no acceleration and no gravity), then it is impossible to tell... this was one of the basis for Special Relativity, that there is no preferred inertial reference frame, thus everyone can say that they are at rest and that everything else is moving. This combined with the constant speed of light (c) in all reference frames leads to the weird implications of Space and Time! So, you have hit on one of the fundamental theories of modern physics!