There is a very interesting article which appears on the Zenit website (See ROME, MAY 30, 2010 - http://www.zenit.org/index.php?l=english) on the subject of how the Church authenticates miracles. It is by Father John Flynn, LC, and the full article may be found by following the above link.
The article gives very useful information regarding the way that the Vatican authenticates miracles. Catholics are accustomed to the idea of miracles generally, and in particular, of people being cured through the intercession of the saints. However many find it difficult to give a reasoned response to the sceptical questions of those around them. Fr. Flynn refers us to a recent book "Medical Miracles: Doctors, Saints, and Healing in the Modern World" (Oxford University Press) by Jacalyn Duffin. She is a Doctor who holds the Hannah Chair for the history of medicine at Queen's University, Ontario, Canada and during her researches she examined 1,400 miracles cited in canonizations from 1588 to 1999.
Her interest in the subject arose when she was asked to examine tissue samples, and some time later, documentation, which were part of a canonization process. She began to realize that these must exist for every canonized saint.Soon after, she researched hundreds of these records and considers that she reviewed between a third and a half of all miracles deposited in the Vatican archives since the rules governing canonizations were laid down in 1588.
Evidence
Many people today are unaware that the regulations that were part of the Counter-Reformation reforms demand a rigorous gathering of evidence and a scrupulous examination of the material by medical and scientific experts. For a healing to be deemed miraculous it should be of an incurable illness and the recovery should be complete and instantaneous.
In respect of suspected miracles the Church always acts as a ‘Devil’s Advocate,’ and relies on a scientific scepticism to test their validity. In the records of the miracles that Duffin examined she found that the religious authorities readily deferred to the opinion of scientists and withheld a judgment until they were convinced that the experts were prepared to label the events as inexplicable.
Duffin noted there is much argument over whether, just because there is no apparent explanation for a cure, it really means that the event is a miracle. For example, one of her colleagues explained that while we may not know the natural explanation, one must exist.
Duffin’s response is that to assert that a miracle simply cannot occur is no more rational and no less than an act of faith than the belief that miracles can happen.
The Catholic Church of course believes in the possibility of divine intervention, but it first uses all of medicine's and science’s resources to remove the possibility of any natural explanations of the cures. In addition, the Vatican does not recognize healing miracles in people who have refused orthodox medicine to rely solely on faith. The intervention of doctors provides objective medical evidence that avoids any possible manipulation of the case in question.
The use of doctors to rigorously sift through the available evidence has increased and indeed, the Church does not rely exclusively on Catholic doctors. The inquiries examined the faith of all witnesses, doctors included. Even where doctors were Catholic many admitted they did not regularly practice their faith. Indeed a couple had been excommunicated, but nevertheless they were not disqualified from being witnesses. Nowadays the medical and scientific expertise of doctors of other faiths, or none, has been given in testimony.
Since the second half of the 17th century, the evidence of the doctors treating the patient has been supplemented by independent medical observers, and today the number of experts consulted in every case matches or even exceeds the numbers of doctors attending.
Ultimately a miracle is only declared when the medical experts admit that they are unable to find any natural explanation as to how a person recovered when the best scientific medicine failed.