A low stage cancer means it hasn't spread and therefore has a better prognosis. A high stage cancer means it has spread and affected other organs. Metastasis may be suspected based on abnormal blood tests and can be identified by medical imaging or exploratory surgery. Imaging is looking at the internal organs without cutting through the skin. These are usually the first choice for looking for metastasis.
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.