Yup... there is pre-visualization and simply just knowing your lenses intimately. I'm not going to lie... it takes practice. You can "know" how a 28mm lens will render the subjects as opposed to 50mm. You can become familiar with what f/2.8 looks like as opposed to f/8. etc..
The other side of the DOF is that SLRs always focus wide-open.... DOF is shallow and you can loose touch with what exactly is in the background. RF not so much.. you can see and react to subjects in the background, just outside the frame, and what is coming/going in/out of the frame.
That's a very interesting perpective on estimating depth of field. Being an SLR user I have become accustomed to working in the opposite direction - seeing the minimum depth of field and visualising what the final image will look like depending on the aperture set. I rarely use the depth of field preview button. I agree that an SLR can be limiting in terms of seeing outside the frame. You can offset this by opening your left eye briefly but unless you have a standard lens fitted it is easy to make your brain hurt by seeing through each eye at a different magnification!
Thanks for the responses.