Web-DVM has set up a dog and cat Symptom Checker. (The term symptom, really does not apply to veterinary medicine, but is used here merely because of the familiarity most people have with the term. Strictly, a symptom is a sensation or change in health function experienced by a person.)
In veterinary medicine, with patients that cannot tell the owner or veterinarian how they are feeling, we look not for symptoms, but instead, signs of disease. Signs your cat or dog may be showing, as opposed to symptoms, are objective indications of some medical fact or quality that are detected by a person or veterinarian.