The tongue scale is a nice piece of equipment.
You can use a bathroom scale on a hard smooth surface. Set it under one end of a double 2 x 6 beam with the other end of the beam on blocking with a narrow pivot point. Set the tongue jack on the wood beam. If your scale weighs to 400 lbs and you think the tongue weight is 900, put the jack down 1/3 of the beam length from the blocking end. The scale should read 300 if tongue is 900. The fixed end is carrying the other 600lbs. If your wood beam busts make it thicker.
If you hear Weight Police sirens approaching hide in the bushes ...