Brain tumors are very hard for vets to diagnose.
They are tough to determine without a CT or MRI. These aren’t available to everyone, since many don’t have access to this technology nearby, and they can be very expensive.
Also, they’re not easy to treat. The age, breed, and sex of a dog all play a part in the treatment of a brain tumor.
The history you give a vet about your dog will help the diagnostic process. Did the issue start all of a sudden or slowly? What about appetite, thirst, and other indications of sickness? Is it worsening? Is there weight loss? When we think about brain cancer, much of the time the dogs will have some sort of warning signs. These include seizures, blindness, etc. (Note that other diseases can also produce these symptoms.)
However, if one side of the dog’s body is acting differently, this is more indicative of a brain tumor. Maybe there is a loss of coordination affecting the left side more than the right side. Or perhaps a pupil is distended or constricted in the right eye though not the left. Most likely the retina (seen by looking in the eyeball) looks different on one side. It’s also possible that the muscles in your dog’s face are weak or sagging.
So if the issue is pointing to the brain in an older dog, that means that it’s asymetrical (having an effect more on one side of the body than the other), it is progressing over the course of time and lab testing rules out the other causes that produce the same symptoms, we are left with a brain tumor as the number one diagnosis.
As far as treatment goes, doing brain surgery on dogs isn’t customarily done because it doesn’t have a high success rate. There are some rare doctors out there that have done it successfully but they’re few and far between.