Maybe TMI for some, but very good article.

http://www.fda.gov/cvm/petlabel.htm

Kestrel's take-home points:
Feed a food that says "Manufactured by" rather than "Distributed by" (which is a "generic", the lowest quality food generally).
Feed a food that says "Animal feeding tests using AAFCO procedures substantiate that (name of product) provides complete and balanced nutrition." Don't feed something that hasn't been proven capable of sustaining good health. Properly formulated food won't kill any animals when tested, BTW, unless there is unanticipated taint as in the Menu Foods case.
Feed a "life stage" food that provides complete and balanced nutrition for a particular life stage i.e. growth or maintenance. Kitten food has too much protein for older animals. Optimum protein and fat for adults or older animals is sub-optimum for young animals.
Beware of any foods containing garlic or onions. All alliums are known to be poisonous to dog and cat red blood cells, and their presence in a food indicates a manufacturer who doesn't know or worse yet doesn't care about harming animals.
Be skeptical if you note the presence of bizarre ingredients like avocado, blueberries, eye of newt, etc. These may attract purchasers but are not a valid indicator of quality or appropriateness of nutrition.

These are basics that I use to pre-screen any diet before I will consider recommending it to a client for their pet.