If the load is being carried on the axle/wheels at a constant speed the force required to keep it moving at that constant speed depends on the rolling friction of the bearings and tires.
For instance if the rolling friction is 0.1 it means you have to apply a force of 50lbs to keep your 500lb load moving at a constant speed. You have two 12" diameter tires in contact with the road so each tire contributes 25lbs. The radius of the tire is 6" so that is the lever arm for calculating the torque which is 25lbs * 0.5ft = 12.5lb-ft.
This calculation ignores the fact that the tire compresses slightly under the load so the radius is a bit less than 6".
Your idea of actually measuring the force required to move the load is a good approach because you do not know what the rolling friction is.