Suppose we have two frames of reference, with one being accelerated (not inertial). This could be you standing on the platform of a train station as a train in front of you is starting. From the train's point of view, you are accelerating, but one obviously knows that it is actually the train that is accelerating, because you feel no force (acceleration) being applied to you. However, if we have two reference frames where we are not a part of either of them, but instead we are merely observing/measuring them, how do we know which one is the non-inertial one? Thanks for any replies in advance.