Assuming your cat is in good health, digestion can be judged by stool quality and volume. Small, firm stools are a sign that her nutritional intake is being put to work, and not passing through as waste.
Digestive upset is one of the most common reasons cats are brought to their veterinarians. Some digestive problems can be a result of food allergies, and may resolve once the cat’s digestive system is given a chance to rest. But more serious conditions could result in weight loss, dehydration and debilitation. Your veterinarian can determine the appropriate treatment.