On a pet food label, the number one listed ingredient is the number one thing in the food. With many pet foods, the number one ingredient, is corn. Now the 2nd trick is to figure out how much meat there really is in the food. Using chicken as an example, there is chicken meal, chicken, chicken digest, chicken by-products. The first 2 are the best.
Most pet foods use by-products.
Chemically, this will analyze out as protein, but obviously your pet can't get much nutrition from this type of protein. Most pet foods are preserved with artificial chemicals. Ethoxyquin, BHT, BHA etc. Many foods have artificial flavorings and colorings. Unfortunately, most won't be found in the store.