Exploratory surgery is the other method of looking for metastasis. It's better in some situations than imaging because you can takes sample of tissues directly to see if the cancer has spread. Once cancer has been identified, graded and staged, treatment options can be explored. A cancer diagnosis in a pet is not a sentence of immediate euthanasia. Surgery is the treatment that's most likely to be curative.
Surgery removes the cancer from the body.
For example, if your pet is diagnosed with a cancer of the jaw, removing the affected portion of the bone before the cancer metastasizes is curative because the cancer is gone. Surgery is only curative if the cancer hasn't spread, or if the other affected areas can be removed surgically. Obviously for something like cancer of the white blood cells, or leukemia, surgery won't work because that cancer can't be cut out. Radiation therapy is another treatment option. Radiation can work well on a local area of cancer that either can't be treated surgically, or was treated surgically and some of the cancer remains in the tissue.