Well, I don't really need an absolute inertial frame, and that is a good thing, since they are hard to come by in the real world. (Lots of approximately inertial frames, none that are exact.)
So if I let our fun push me into a rhetorical corner, I would say they are all rotating. The inertial ones are the ones where the rotation rate is zero. And operationally (or at least thought-operationally) you determine if you are in a zero-rotational-rate frame by measuring the Coriolis force. You can't use the centrifugal force, because locally you cannot tell the difference between it and the reaction to a linear acceleration.