First of all, a pet owner should know something about pet-food labeling and be able to examine an ingredient list to judge the quality of a food.   Pet-food manufacturers must by law adhere to precise rules and definitions in the labeling of their products.

Ingredients are listed in order of descending content in the food. In other words, the first ingredient listed is the predominant ingredient, the second ingredient, the next most predominant, and so on.

Animal-derived protein is the most expensive ingredient in pet food and the ingredient most likely to be less-than-wholesome. It is perfectly legal for the rendered hides and hooves of slaughtered animals or the rendered carcasses of road-killed animals or diseased animals  to be added to pet food. Such components may be identified as "meat and bone meal" or "animal by-product meal," on pet-food labels.  These protein sources are the lowest quality legally available for pet food, and are highly likely to be less-than-wholesome.  Not only is the source of "meat and bone meal" suspect, the rendering process itself may destroy many valuable nutrients.  The word "meal" on a pet food label denotes an end-product of rendering.

The words "beef", "chicken", or "lamb" denote the highest quality protein sources in pet food. When these words appear on ingredient lists, the protein source must be the "clean flesh" of "slaughtered animals".  Again, not necessarily the most appetizing cuts, but probably better than left-over carcasses or by-products.