Start with the digestive signs you can see daily
The gut usually files the first complaint. Stools that become softer, bulkier, or more frequent than your dog's normal, extra gas, audible gurgling, lip-licking or grass-eating, and the occasional vomit after meals all suggest the current food is not suiting him. Compare against his own baseline, not an ideal picture: a dog who has always produced two firm stools a day and now produces four soft ones has told you something, even if each individual stool does not look dramatic. Appetite changes count too — a dog who suddenly leaves food he used to finish, or who eats and then seems uncomfortable, is giving usable evidence. One off day after raiding a bin means little; a pattern across a week on the same food means a lot.
Skin and ears are the slower second signal
Foods that disagree do not always stop at digestion. New itching without fleas, recurring ear redness or head-shaking, paw chewing, a duller coat, or flaky skin that appeared in the weeks after a diet change can all accompany a food sensitivity. These signs build gradually, which is why owners often blame the season or a shampoo first. Skin signs are not proof on their own — parasites, pollen, and infections produce similar pictures — but when itching follows a food change and no other change explains it, the food belongs on the suspect list to discuss with your veterinarian.
Use timing and one-change-at-a-time testing
The most reliable home detective work is a timeline. Note the day a new food, treat, chew, or topper started, and the day signs began. Intolerance-type reactions often show within days; sensitivities behind skin signs can take longer to build and longer to fade. If signs are mild, returning to the previous food that suited him and watching for improvement over the following days is a reasonable first step, and reintroducing the suspect food is not necessary just to prove the point. Change only one variable at a time: if you swap the food, the treats, and the chew in the same week, you will never know which one mattered. Keep treats plain and minimal while you investigate, since a handful of rich extras can mimic a main-food problem.
Know the signs that skip home testing and go to the vet
Some presentations should not be watched at home. Repeated vomiting, diarrhoea that lasts beyond a day or two, blood in stool or vomit, a swollen face or hives, difficulty breathing, refusal of all food, marked lethargy, or weight loss all warrant prompt veterinary care, and puppies, seniors, and dogs with existing conditions have a lower threshold still. Take a photo of the food packaging and bring your feeding notes, because the ingredient list of everything he has eaten — meals, treats, chews, and scavenged extras if you know them — is exactly what the vet will ask about. Only a veterinarian can work through whether signs point to a sensitivity, an infection, or something unrelated to food entirely.
Turn scattered impressions into a usable record
Owners usually arrive at the clinic saying he has been a bit off for a while, because nobody wrote anything down. A simple written log — what was fed each day, the stool quality, any itching or vomiting, and how his appetite and energy seemed — transforms that conversation. Patterns that memory smears into a blur become obvious on paper within a fortnight.
Storing that feeding history in KinsLe's Pet Passport keeps the record with your dog's other health information in one portable packet, so the veterinarian sees what he ate and how he responded, visit after visit.