In our physics course, Newton's first law was given as a definition of inertial reference frame. Now in order to use it we need to take some object which has zero interaction with other objects. But now in order to check it, we need to know which forces are acting on an object. But how can we be sure that the fact that an object, with zero net force on it, is accelerating because we are in a non−inertial reference frame and not because of the fact that we haven't accounted for some "real" (as opposed to those called "fictitious") forces. And in general, how can we determine that the force that acts on an object is "real" and not "fictitious" ? E.g. how can we determine that electromagnetic force is "real" and not "fictitious" ?