There are two basic ways to tell when pizza dough is properly mixed when the dough will be managed through a cold fermentation period of at least 24-hours in the cooler/fridge.
The first, and easiest to describe is to watch the dough as it mixes, at first you will see a dark colored dough with a rough appearance. As the dough continues to mix/develop the color will change to a brighter, yellowish color and the dough will begin to take on a smoother appearance in the mixer. This is an indication that you are getting close to correct development. Continue mixing just until the dough develops a smooth skin in the mixing bowl, at this time the dough is best described as having a smooth, satiny appearance. You're now done mixing.
The other method is to take a piece of dough, about the size of a large hen's egg, and loosely form it into a ball, bring your hands together with your finger tips curled inward. Your finger tips should now be touching, position the dough ball on your fingertips and bring your thumbs down onto the dough ball, locking it in place on your fingertips, now roll your hands apart as if trying to bring the second joint of each finger into contact with one another, the dough will stretch between your locked thumbs, check the dough to see if it tears, if it tears the dough will require additional mixing time, but if it stretches without tearing the dough is properly developed. Like I said, this is not the easiest procedure to describe, but it works well as does the first method described above.
Tom Lehmann/The Dough Doctor