As a general rule, you very often "get what you pay for" with commercial pet food kibble. The cheaper the food, the cheaper the ingredients. The cheaper ingredients, the less healthful the food. Keeping your pet healthy with a good diet and exercise will help to reduce veterinary bills in the long run.
Purchasing the best food you can afford is a first step to "getting your money's worth" for your pet's nutrition.

Shop at store that you know sells a high volume of quality pet food or is diligent about checking stock/replacing out-of-date stock. This way, you can be assured that the food is rotated often and is as fresh as possible. Also, get into the habit of checking the "use by" dates on the bags prior to purchase. Look for any tears, surface mold/dampness marks, or debris on the bags and select an intact one with the freshest date.

I have been in a hurry running errands; stopped at the pet supply store, grabbed my preferred pet food, and headed home, only to discover one or more of the above problems (bad packaging, close to the "use by" date, etc.) once I am home. Making a habit of inspecting these things will save both time and money, and be a healthy choice for your pet.

If running errands on a hot day, try to save the pet food purchase until the end of the day. High temperatures inside of the car (while it is parked and other errands are completed) can spoil food quickly; even if the "sell by" date is far into the future.

If you notice your pet not wanting to eat their regular food or experiencing gastric distress or excessive flatulence, check your pet food label for freshness and make sure that the food hasn't spoiled. As always with a sick animal, speak to your veterinarian as soon as possible in the event of illness. If you notice anything amiss with the food packaging, be sure to mention this to your veterinarian as well.