What are some criteria the medical community uses to determine if one is “brain dead”?

A. Prerequisites.  Brain death is the absence of clinical brain function when the proximate cause is known and
demonstrably irreversible.
1. Clinical or neuroimaging evidence of an acute CNS catastrophe that is compatible with the clinical
diagnosis of brain death
2. Exclusion of complicating medical conditions that may confound clinical assessment (no severe electrolyte, acid-base, or endocrine disturbance)
3. No drug intoxication or poisoning
4. Core temperature ≥ 32° C (90°F)

B. The three cardinal findings in brain death are coma or unresponsiveness, absence of brainstem reflexes, and
apnea.
1. Coma or unresponsiveness--no cerebral motor response to pain in all extremities (nail-bed pressure andsupraorbital pressure)
2. Absence of brainstem reflexes
a) Pupils
i. No response to bright light
ii. Size: midposition (4 mm) to dilated (9 mm)
b) Ocular movement
i. No oculocephalic reflex (testing only when no fracture or instability of the cervical spine is apparent)
ii. No deviation of the eyes to irrigation in each ear with 50 ml of cold water (allow 1 minute after injection and at least 5 minutes between testing on each side) Brain Death
c) Facial sensation and facial motor response
i. No corneal reflex to touch with a throat swab
ii. No jaw reflex
iii. No grimacing to deep pressure on nail bed, supraorbital ridge, or temporomandibular joint
d) Pharyngeal and tracheal reflexes
i. No response after stimulation of the posterior pharynx with tongue blade
ii. No cough response to bronchial suctioning

3. Apnea--testing performed as follows:
a) Prerequisites
i. Core temperature ≥ 36.5°C or 97°F
ii. Systolic blood pressure ≥ 90 mm Hg
iii. Euvolemia.  Option: positive fluid balance in the previous 6 hours
iv. Normal PCO2. Option: arterial PCO2 ≥ 40 mm Hg
v. Normal PO2 Option: preoxygenation to obtain arterial PO2 ≥ 200 mm Hg
b) Connect a pulse oximeter and disconnect the ventilator.
c) Deliver 100% O2, 6 l/min, into the trachea.  Option: place a cannula at the level of the carina.
d) Look closely for respiratory movements (abdominal or chest excursions that produce adequate tidal volumes).
e) Measure arterial PO2, PCO2, and pH after approximately 8 minutes and reconnect the ventilator.
f) If respiratory movements are absent and arterial PCO2 is ≥ 60 mm Hg (option: 20 mm Hg increase
in PCO2 over a baseline normal PCO2), the apnea test result is positive (ie, it supports the diagnosis of brain death).
g) If respiratory movements are observed, the apnea test result is negative (ie, it does not support the clinical diagnosis of brain death), and the test should be repeated.
h) Connect the ventilator if, during testing, the systolic blood pressure becomes ≤ 90 mm Hg or the pulse oximeter indicates significant oxygen desaturation and cardiac arrhythmias are present; immediately draw an arterial blood sample and analyze arterial blood gas. If PCO2 is ≥ 60 mm Hg or PCO2 increase is ≥ 20 mm Hg over baseline normal PCO2, the apnea test result is positive (it supports the clinical diagnosis of brain death); if PCO2 is < 60 mm Hg or PCO2 increase is < 20 mm Hg over baseline normal PCO2, the result is indeterminate, and an additional confirmatory test
can be considered.

Source:http://www.aan.com/professionals/practice/guidelines/pda/Brain_death_adults.pdf