Can you tell if your pet is ill simply by looking at them? In nearly all cases, the answer to this question will be no. Identifying that your pet is sick and discovering the underlying cause of their illness can be a complex process that looks at many factors – any symptoms your pet has, how they are behaving and how their body looks. Your vet will also need to take into account any internal changes that are happening and that aren’t visible to us. This is because many health problems cause internal changes to things like your pet’s blood, urine or organs. The only way of identifying these changes is to perform diagnostic testing.