>>5
Certainly not possible as long as you don't have an exact definition for pain on the cellular level.
Neuron signal strength, you say? At which point would you measure it? The last neuron before entering the brain? Well, the brain can't be treated as a magic black box either. But once in the brain, do some signals represent pain and some not.. how exactly should they differ? You can also experience "emotional pain". Is it biologically equivalent to "body pain"?
Certainly, for the pain to be real, it must affect the subject's behavior in one way or another. I'd imagine an objective measure of pain being related to the strength of motivational influence of the signal.