I like to check the bottom, then the top edge, and the cheese color along with the toppings. Each of us has a crust color or baked characteristic that we are looking for so you really can't apply any specifics, but like Walter said, it soon becomes pretty intuitive and a good oven tender can pretty well nail it "spot on" every time. Then you have the shops using the most popular type of ovens, the air impingement oven. With these ovens you place the dresses dough skin on one end of the conveyor, and a fully baked pizza emerges from the other end. Since the human element has been removed these ovens depend upon consistency to provide a decent bake, by this I mean that you must have a consistent product going into the oven to have a consistent pizza coming out of it. Dough temperature is critical, as is the temperature of the toppings, dough thickness also plays into the equation, and it goes without saying that all like pizzas must be baked on a like platform (pan, screen, etc.). These ovens are set up to bake pizzas for a specific shop, once set up they pretty well run themselves, as long as the operator continually provides a uniform and consistent product to the oven. If that doesn't happen, things go to heck in a hand basket pretty fast.
Tom Lehmann/The Dough Doctor