My experience, based on reading and on talking to the vet and on personal experience with dogs and other animals (the general rules are the same, regardless--feed the animal the diet that is closest to what they would eat in nature, since that's what they are designed to eat) is that:
You need to read the label on the pet food. The first two ingredients should be meat (and preferably not "meat by-products") for dogs & cats. The other ingredients should be veggies, with little to no grains. The further down on the list the grains are, the better. We found out the hard way that our one dog has an allergy to beet pulp, a common filler. Also, keep in mind that they need to update their labels every 6 months---so they could change their formula, but you wouldn't know it unless you checked the bag, and even still, your dog could be eating the "new" formula for months without you being able to find out.
There are lots of dog foods that are on a par w/ Science Diet that are less expensive and just as good. We are currently using Premium Edge (we had been using Nature's Recipe, but they upped their prices AND made the bags smaller). I used the www.petfooddirect.com website because they list literally hundreds of kinds of kibble, and they post the nutritional information right there for you to view. Then, I narrowed it down to 5 or 6 brands and went price shopping at the local stores to find the best deal.
I've had to do the above process 3 times in the 10+ years we've had our current dogs, and 3 times when my cat was alive (he lived till age 19). I'm very cost conscious, and that is often what drove me to change.
Once you do switch your dog's food, do it gradually over the course of a week or so. And then monitor how the dog acts and looks on the new food.