Honestly, I feel bad for the average pet owner. Don't trust the vets, they're in bed with the companies. Vets don't know much about pet food anyway. (how would it be if I were to tell people not to trust their doctor all the time - health care would be a mess!) Don't trust the shelter you got the pet from, they're in bed with them too. It's "common sense" to not trust the guy at the store, who (of course) is trying to sell you his products.
How are people supposed to know?
Most people I know will trust their vets (their pet's doctor!).