I have been working in vet offices for a few years now. Ol'Roy is like eating McDonalds on a day to day basis. I would also suggest to stay away from Iams, Science diet, and most foods you buy at the grocery store. The best brands of food that I have come across, and are easy to find are Royal Canine and most natural dog foods.
When looking for ingrediants, vitamins E, C and beta carotene's are very important. Generally you want to stay away from any by-products.