Observationally, we find that this Newtonian or inertial frame is one in which the distant galaxies are not rotating. Some people say that the water goes down clockwise in the Southern hemisphere and anti-clockwise in the Northern hemisphere. I am tempted to suggest that these people have never, or very rarely, looked. I have never noticed a correlation with the hemisphere. Here is an experiment for you to try.
Part fill it with cold water.
Once you have done so, pull out the plug and observe the direction of the water flow. Then part fill it with hot water. Visitors are invited to start it swinging in a plane that is accurately defined by a fixed vertical wire and a vertical line on the wall. It is easy to see when the cable of the pendulum is swinging in this plane. The user then sets an indicator to show the time when it started.