Determining Load Force While Towing

I would say your problem here is knowing the resistance of whatever you are towing.

Using F=ma you can see that the force would be zero if you are towing at constant velocity in ideal conditions. ie no frictional losses etc.

The tyres, bearings etc are gonna cause frictional losses and these are individual to each situation.

What shape is the trailer (or airplane)? Wind resistance depends not only on the shape but also what speed you are towing it at.

Maybe there is a general rool of thumb but I don't know one.

You are right on the resistance issue. The only thing that I can think of that comes to mind is the tow hitch numbers. The tow hitch should carry 10% of the trailer's weight. But the pulling load must be higher than that. If the hill were steep enough, or there was a wheel stop against the trailer wheel, you could have to pull the entire weight of the trailer.

"How do i determine the load force while towing."

A load cell or something similar would eliminate all doubt and be closer to the truth.