How can a doctor tell that the leg of a diabetic will need to be amputated?

Last resort: The complications and potential problems make amputation always the choice of last resort. Best indications are intractible pain and a functionless lower extremity that places the patient at highest risk for long term morbidity or mortality. A non-healing wound is never an indication for amputation by itself. As long as the leg is usable, a smart doctor will avoid amputation.

Many possibilities: Some indications for performing an amputation may include: cancer of the affected part, infection, sever vascular disease, gangrene, and sever structural deformities.