I have never checked, I only assumed it was accurate enough for my purposes: if there is very low or zero backlash in all positions of a worm drive table, and no tight spots/loose spots, then its good. Anything less than an almost perfect gear will not operate with low backlash as you will have to give the worm and gear enough clearance to get past the tight spots.
Now if this is not good enough, then you probably should not have built a rotary table with gears of unknown quality. To do better, you need accurately hobbed gears and ground thread worms that have verified runout that is less than your specification.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)