I found an e-book that explains some of what the AAFCO is looking for in feeding trials. It seems to center mostly around nutrient density, digestibility and the amount of nutrients consumed.
What's odd to me is that labels generally only list crude fiber, fat and protein and don't address bio-availability of the nutrients. I guess the consumer needs to look at the suggested feeding amounts for a given weight, the idea being the less food needed for a given weight would be a more nutrient dense food. This can be confusing, too, due to variances in kibble density and shape. A cup of food 'A' may weigh more or less than a cup of food 'B' so it's easy to get confused. Dogfoodanalysis offers some more info but it's still hard to get a complete picture. I think it's important to remember that AAFCO certification only means the food is meeting the MINIMUM nutrient requirement for the food and optimum nutrient levels are often much higher.
My gut feeling is that most companies don't use inhumane lab testing these days. I do remember seeing commercials, though, where companies would say things like"feeding our food results in an average X% more muscle mass" or "X% stronger bones". These types of claims you can be sure resulted from disgusting lab tests.
Here's the link to the e-book that explains some of the AAFCO's methodology.
Nutrition for Veterinary Technicians ... - Google Books