An enlarging lump
Just as on yourself, if your dog has a lump that’s getting bigger or changing in shape or texture (especially if there are multiple lumps), you should see your vet. He or she will probably want to take a biopsy.
Swollen lymph nodes
Lymphoma is a kind of cancer that can cause lymph nodes (located throughout the body but most easily detected behind the jaw or knees) to swell. While other illnesses commonly cause this as well, if you notice swollen lymph nodes, you should always take your dog to the vet. Anything that causes swollen lymph nodes needs to be treated. If your vet suspects cancer, however, he or she will likely recommend a biopsy or cytology of the enlarged gland.
Chronic weight loss
Unless your vet has your pup on a diet, weight loss isn’t a good sign. The good news is that loads of things can cause your precious pooch to lose weight, but many cancer patients (human and otherwise) do experience weight loss.
If your dog’s belly rapidly becomes enlarged, it’s possible there’s some sort of mass (i.e., tumor) in the abdomen, or it could be a sign of bleeding in the area. Don’t confuse this with slow swelling of the abdomen, which likely indicates you’ve been overfeeding Fido. Your vet will probably order a radiograph or ultrasound.
Unexplained bleeding or discharge
If you see any unexplained bleeding from the mouth, nose, vagina, penis or gums, you should have your dog examined by a vet. It’s possible this could be due to some sort of trauma, so think about what you and your dog have been up to that could lead to an injury like that. It’s also possible that if your animal is still a puppy or very young, it’s a bleeding disorder of some kind. If you see any discharge you can’t explain, that could also be a sign of trouble. In any case, a vet needs to make the determination, especially if your dog is older.