Different Ways of Taking the Evidence, I

 A friend tipped me off the other day to a book by a Protestant missionary, Don Richardson, titled Eternity in Their Hearts.  The basic thesis of the book is that a variety of (sometimes obscure) cultures throughout the world possess some version of monotheism, often combined with some version of the story of the Fall.

It's not unusual to come across analogies between Christianity and pre-Christian religions; what struck me in this instance was the fact that, while the similarities are frequently taken as an argument for debunking Christianity, Richardson takes the same evidence as supportive of Christianity.

Same evidence, different conclusions based on different priors.  So how does one know which conclusion is more likely to be true?  Or, to ask an even less ambitious question, how does one tell which way the evidence actually weighs?