As dog food ages, the oils in the fat turn bad and the nutrients degrade. Not only does your pup receive less nutrition than if he ate fresh food, he may become sick. If you suspect the food has turned, don't force your pet to eat it.
Sniff the dog food. It develops an off smell that indicates the food has aged poorly and may have gone rancid. Dog food may not smell wonderful on a normal day, but if it smells downright bad, toss it and find fresh food.
Inspect the kibble. Does it look moldy or appear to be coated in dust? If yes, then the food is not fresh and may have gone rancid.
Look for changes in your pal's bathroom habits. Diarrhea can be a sign the food is rancid.
Watch your pup for other clues. If your dog normally eats his food, but suddenly becomes reluctant to eat or outright refuses, he may be trying to tell you it's gone bad.