I posted this in another thread...
IMHO, it is fairly comprehensive and explains what labels mean. That post is actually about dog food but the same goes for cat food.
The first thing I look for is that the main (first) ingredient is real meat. Then I see if it has been tested and approved by AAFCO and being "complete and balanced."
Actually, the first ingredient being a "meal" is probably going to be better than a "meat". But the link above explains that too. AAFCO guidelines are so lax that even foods like Friskies.
Most states have adopted the guidelines as law so looking for an AAFCO approved food does little to ensure quality.
Like Lixx said, Science Diet isn't the worst...but it's not the best either. For the same price, you could be feeding a better food.
Adult Original ingredients...
Chicken by-product meal, corn meal, brewers rice, animal fat (preserved with mixed tocopherols and citric acid), corn gluten meal, chicken liver flavor, taurine, preserved with mixed tocopherols and citric acid, minerals (potassium chloride, calcium sulfate, salt, ferrous sulfate, zinc oxide, copper sulfate, manganous oxide, calcium iodate, sodium selenite), rosemary extract, beta carotene, vitamins (choline chloride, vitamin A supplement, vitamin D3 supplement, vitamin E supplement, L-ascorbyl-2-polyphosphate (a source of vitamin C), niacin, thiamine mononitrate, calcium pantothenate, riboflavin, pyridoxine hydrochloride, folic acid, biotin, vitamin B12 supplement).
IMHO, by-products and corn have no place in pet food. Corn and brewer's rice are low quality fillers. And animal fat is a collective feed term that can refer to any combination of animals...this means that the composition of this ingredient can change in each bag that you buy (the company does that on purpose to save money).
Better choices in the same price range are Nutro Natural Choice (contains corn but no by-products) and Precise (contains corn but no by-products). Even better choices, but more expensive are Innova, Wysong, Wellness, Felidae, and many more.