how does the coriolis effect provide evidence that the earth rotates?

To answer this requires being able to construct a very simple-minded example of how the Coriolis effect works. Suppose you are an identifiable clump of water floating in the ocean, not actually in contact with any solid surface. And let's say you are floating poleward. You have a certain eastward momentum inherited from your immediately past surroundings. But since you are floating poleward, the eastward momentum of your immediately past surroundings is greater than that of your immediate future surroundings. So an outside observer would see you veering off towards the east. Similarly, if you had started floating towards the equator, an outside observer would see you veering off towards the west (because your eastward momentum wouldn't be enough to "keep up" with your new latitude). What we actually see in major ocean basins is that the poleward currents (Gulf Stream, Kuroshio, Agulhas Current) lead into broad eastward flows, and that the equatorward currents (Benguela, Humboldt, and California currents) lead into broad westward flows. This supports the notion -- well known from other evidence!! -- that the earth is rotating eastward.