If your fuzzy is unfortunate enough to get cancer, chances are it will get Lymphosarcoma. It's better known as Lymphoma in ferrets. Often, there are very few early symptoms of this disease. Some possible symptoms could be weakness, diarrhea, weight loss and swollen lymph nodes. Your ferret may also experience difficulty breathing or develop a cough. There really is only one way to determine whether it's cancer though. Your vet will need to do a complete exam and blood work up. It usually strikes ferrets who are four years of age or older, however there is also a form of this cancer that strikes young ferrets as well.
As in humans, fuzzies with cancer can be treated with chemotherapy. Success rates vary though. Sometimes the remission lasts 6 months or more, other times only a couple of weeks. However, this treatment is usually not curative. Usually though, the lymphoma will have spread throught your ferrets' little body long before it's diagnosed and can be treated. If the cancer has spread, your vet will be able to tell you what options you have to make your fuzzies life a little more comfy for what time he has left.
Yearly vet visits are a great way to stay on top of your ferrets' health. With luck, you might even be able to catch the lymphoma early on. In any event, if your fuzzy has been under the weather for a while, make sure your vet does a complete blood work up