A miracle that qualifies for sainthood is a miracle occuring after the death of the candidate saint, as a result of a prayer to him. The candidate saint then intercedes from Heaven and the Holy Ghost performs the miracle. The miracle has to be objectively verifiable, typically, a healing. No medical treatment must have been applied that could have been the cause of healing. Presently, many are praying to the late Pope John Paul for healing and there are reports of some possible situations under considerations, as there is a strong belief that he is a saint. At the same time, the fact that a sick person got healed after a blessing of the late Pope when he, the Pope, was alive, do not qualify.
There is also a requirement of martyrdom, sometimes waived. Since the late Pope did not die of his wound, it is not clear if he can be considered a martyr, although we now know tha the was in constant pain following the assassination attempt.
Thanks for the info. I'm a little intrigued by the "objective verification" part. I think we could agree that it happens all the time that people are "inexplicably" healed by God, i.e. with no special medicine, etc. Some of these people prayed to a Saint or Saint candidate for help, and some did not. Therefore, how is it objectively determined that the healing was a direct result of the candidate's intercession, when it could just as easily have been a coincidence, i.e. that God had already decided to do a healing with or without the intercession?
That leads me to another question. If it is proper to pray to a candidate Saint for intercession (and some candidates get turned down), then is it also proper to ask for intercession from any who have passed away, such as deceased relatives, etc.? If "Yes", then that would seem to be quite a roll of the dice, depending on where the deceased person actually is according to your beliefs. Do people feel "safe" in praying to John Paul II because everyone assumes he is already in heaven?