It is not a big deal to do a calibration check. You hang a weight on an arm and check to see if the torque reading is where it should be.
I check calibration on mine every time I turn it on. Ask them to check it in front of you,if they won't look for someone else.
On Superflow dynos ther is a dial by the operators left knee to make minor adjustments. An unscrupulous operator can turn the dial up to add power and make you think he is a great tuner. Make sure the torque reading is zero or maybe Plus or minus a lb-ft of torque with the engine not running
Keith