You can look at the correction factor, usually if the weather is good and elevation is between 0 and 2000ft. the CF will be 1-1.04.
You can ask them when the last time they calibrated it...or if you business means alot to them they may calibrate it in front of you?

As delivered, our DTS reads slightly low compared to what they recommend, and much lower than what a Superflow CF would read but everytime we check calibration it's the same, so I feel no need to change it. Ours is less than 1/2% off.

It's also good to find others who have used the same dyno as you, and find out how they run at the track vs. the power they made.
Brian