Measuring the force applied to a measured moment arm will give you what is needed to compute the torque.
I think the simplest way is to use a servo horn and a kitchen scale (if you already have one).
On the servo horn, measure the distance from the axis to the mounting hole you'll use. When that arm of the servo horn is horizontal, your measured distance will be the moment arm for the torque.
To measure the force, to use an electronic kitchen scale to measure how much lighter a weight gets when the servo tries to lift it.
Choose a weight a little too heavy for the servo to lift. With the weight on the scale, use string or something and rig the servo to try to lift it at "arm's length" -- that is, with the servo horn arm horizontal. With the weight on the scale, but servo not lifting, zero (tare) the scale. As the servo struggles to lift the weight, whatever force it exerts will reduce the scale's measured weight, directly reading the force you want, albeit negative.
If you are measuring your only servo, consider that some servo designs cannot tolerate stall. Overheating and broken gears are the symptoms of engineering to minimize weight and manufacturing cost.