Proteins are made up of amino acids. There are 22 of them, but 11 are not manufactured in sufficient quantity by the dog's body. These 11 are called essential amino acids, since it is essential they be added to the diet. Some proteins will supply more of one amino acid than another. That's why a mixture of protein sources is important.
Every pet food sold in the U.S. and Canada is labeled with certain nutritional information.
This label is entitled the Guaranteed Analysis. Sometimes, ash, calcium and phosphorus are listed, too. Let's look at each of these, and see what they tell you about the food. The number given for crude protein is a minimum. So, at least that percent of the food will be crude protein.