There appears to be a couple of tests doctors current use....
Confirming Brain Death
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Many physicians request additional, confirmatory tests before pronouncing brain death. The two most common are the electroencephalogram (EEG) and the cerebral blood flow (CBF) study.
Once a patient fulfills this medical criterion, nothing can be done for the patient. The patient can still be maintained physiologically on the ventilator, but there is no hope of any kind of recovery, even to a state of irreversible coma
Families of a brain dead patient must, by federal regulations, be provided the option of organ donation. If the family consents, the regional organ procurement organization is involved. If the family refuses, the mechanical ventilator, medications and fluids are discontinued, after which the heart stops
http://science.howstuffworks.com/environmental/life/human-biology/brain-death5.htm