How Many Pounds Are We Pulling

In researching Hitch Capacity here, most of those discussions focus on tongue weight. I would like to have some estimate of how many pounds of pressure is exerted when pulling a toad or trailer. I can, with not too much effort, push or pull my toad by myself on level ground but not up a 6% grade. That's not 60 MPH either. All this comes to mind when working on winch capacity to load a toad on a trailer. Minus a "load cell", how might one compute the amount of "pull" in pounds to move a 4500# vehicle (tires properly inflated)?