Because cancer symptoms don’t always show up right away, it can be difficult to tell if your dog has cancer, says Dr. Lachowicz. Some clues, however, include: Strange lumps or bumps, like swollen lymph nodes under the neck or groin, or lumps under the skin Vague signs of illness, such as decreased appetite or weight loss, coughing or sneezing, vomiting or diarrhea, or difficulty urinating Behavioral changes, like limping, lameness, lethargy or reduced activity, particularly in younger dogsIf your dog shows any of these symptoms, take them to a veterinarian for an exam.