IIe and IIc have a vsync status bit that you can poll, but IIRC the polarity was flipped between the two models and there are other quirks that can interfere with using it, so it was neither recommended nor popular.
it's more complex than that, I think IIe and IIgs have the same register but polarity reversed, the IIc has a weirder interface that generally involves setting up an interrupt through the mouse firmware