I recently came across a series of articles from various sources that either claim that Saint Francis Xavier's miracle claims are historically verified, or that they are not. Some sources offer examples of testimony given in his favor, such as that of a sailor who witnessed him turn sea water into fresh water. Needless to say I thought it was kind of strange that such wild claims were being made, and wanted to ask exactly how historians go about evaluating these sorts of claims.
http://www.therealpresence.org/archives/Miracles/Miracles_005.htm
http://www.debunking-christianity.com/2008/08/xavier-and-evolution-of-legendary.html
http://www.miraclesofthechurch.com/2010/10/raised-from-dead-saints-who-brought.html
Historians don't evaluate the historicity of the supernatural, they may record that such claims were made-- but there's nothing in the historian's toolbox to say "magic really happened". There is historical investigation of _accounts_ of miracles, how hagiographies are written and and so on, but none of that should be taken as historical validation for supernatural claims.
Such claims belong to believers, not to history. Just as physics can't offer evidence for the existence of ghosts, historians can't prove the Resurrection.
So, for example, we could investigate the story of St. Guthlac's belt (he cured a madman possessed by the devil, so the story goes, a demon flew out of the man's mouth); we can speak to the historicity of exorcisms as rituals, but not of the demons they purportedly cast out.