In a pinch you can get a confirmation of whether or not your driver is calibrated by using a screw tightened firmly into a threaded hole on a lever.
Support the lever from the screw as a pivot point and rest the end of the lever on a kitchen or postal scale.
Using the difference in scale reading between the lever resting on the scale and the lever forced down by screwdriver torque and multiplying by lever length you can get an approximate idea of the torque being applied.
Another way to do it would be to hang weight from the lever in a way that you can adjust its position to get a balance and then measure the distance from screw to where the weight is hanging.