I'm not sure that they do know you don't feel anything during the ops. You're brain 'dead', by some definition, but they don't know if you can feel pain. I read a Nature letter saying there needed to be more research in this area.
There are several tests which are done:
The pupils are fixed and do not respond to sharp changes in the intensity of incident light. (mesencephalon, II and III).
? There is no corneal reflex (pons, V and VII)
? The oculo-vestibular reflexes are absent. No eye movements are seen during or following the slow injection of at least 50mls
of ice cold water over one minute into each external auditory meatus in turn. Clear access to the tympanic membrane must be
established by direct inspection. (pons, VIII, III, IV, VI)
? Access to the above reflexes may be prevented on one or other side by local injury or disease but this does not invalidate
clinical testing. In the case of bilateral injury or disease, ancillary testing should be considered.
? No motor responses within the cranial nerve distribution can be elicited by adequate stimulation of any somatic area, e.g. by
supraorbital pressure and pressure applied to the nail bed of a finger (the latter may be contra-indicated by a spinal injury).
Care must be taken to distinguish central response from primitive spinally-mediated reflexes that can be ignored in this context.
? There is no cough reflex response to bronchial stimulation by a suction catheter placed down the trachea to the carina, or gag
response to stimulation of the posterior pharynx with a spatula (medulla, IX, X). (C&P from NHS organ donation article).
I can attest to this, I sat and watched the doctors carry out all these tests on my fiance when he died. He was on the Donor Register and to not honour his wishes would have been wrong of me. After all the tests had been carried out, he was kept on oxygen and then the tests started to ensure that he could donate his organs. I do know of one life extended through his donation.
After my daughter was born, I put myself on the Donor Register.