How to measure torque, when applying force on a leaver arm?

It might be easier/faster to just do this by hand. Find the max distance from the pivit joint to the load and use the standard equation torque = force x distance. No need to get too complicated. Do not forget to include the requirements to accelerate the load (how quickly you want to get it moving), the mass/inertia of the arms, and any friction from the pivot joints/bearings.

For instance, if each link is .5 meters, and the arm can stretch completely horizontal (worse case), the torques needed would be:

(1kgm @ gravity = 9.8N)

Joint 1: 19.6N X .5m = 9.8 Nm torque

Joint 2: 19.6N X 1m = 19.6 Nm torque

Joint 3: 19.6N x 1.5m = 29.4 Nm torque

Joint 4: 19.6N x 2 m = 39.2 Nm torque

Keep in mind that would be a static torque to hold the load with a zero mass arm. Since each arm segment will have a mass associated with it, and this needs to be included as well. Assuming each arm has a mass of 5 kg, and is evenly distributed, the worse case equation would look like this:

Joint 1: (19.6N x .5m) + (49N X .25m) = 22.05 Nm torque

Joint 2: (19.6N x 1m) + (98N X .5m ) = 68.6 Nm torque

Joint 3: (19.6N x 1.5m) + (147N x .75m) = 139.65 Nm torque

Again, that is static torque to hold the load against gravity, not to accelerate the load. If you need to accelerate the load at 2 m/sec, then the worse case Newton force would be 11.8N (9.8 N to overcome gravity, plus 1 N per kg mass per meter/sec acceleration required).

Also, don't forget braking, or else the load will just keep moving past your required position. Either the motor or brake system will need to apply a certain braking torque to slow down the arm and stop the movement.