You should be able to calibrate or at least check the calibration yourself. Here is how.
1. Hold the torque wrech in a vice horizontally by the 1/4 or 3/8 lug.
2. Mesure the distance from the lug to a position on the handle lets say your measurment was 10 inches from the lug to the handle position.
3. with a string or chain hang weight from the handle at the measured position. Let's say that you hang 5 lb
4. Your torque wrench should show the torque created by hanging this weight. In this case the torque wrench should show 10 in X 5 lb = 50 in lb.
You can use what ever weight or distance you want to do the calibration.