One option for you if you are willing to spend ~ 100 dollars is a particle brake. I just saw a 15 in-lb particle brake on ebay for 99 dollars. If you search, you may be able to find a better deal. You can couple a particle brake to your shaft and the torque that the brake applies is proportional to voltage. Their torque is usually pretty linear with respect to voltage.
If you can't spend that much, then I don't have a lot of options for you. But there are some checks you can do. Start with checking winding resistance. Measure the resistance between any 2 of the 3 motor leads and make sure it matches the manufacturer's spec. Stall torque is proportional to resistance, so this is an important value. I would suggest using a strobe or motor tachometer (~ 30 dollars). Measure no-load speed and current. Attach your motor to another motor and back drive the motor. Take an oscilloscope and look at the waveform between any 2 of the 3 motor leads. Measure the peak voltage and record the speed at the time of the measurement. The speed (in RPM) divided the peak voltage should give you the motor's Kv value. The peak voltage divided by the speed (in RPM) should give you the motor's back-emf constant. Even more important is if you take the peak voltage divided by the speed (in rads/sec), then you get a back-emf constant in Volts/(rad/sec) which is equivalent to your torque constant in Nm/A. From your no-load current and your torque constant, you should be able to estimate what your torque is based on the current draw of your motor.