What is the test for credibility in a witness case?

I am guided by the dicta in Faryna v. Chorney[3] which, while decided many years ago, offers guidance on the difficult task of credibility assessment: The credibility of interested witness, particularly in cases of conflict of evidence, cannot be gauged solely by the test of whether the personal demeanour of the particular witness carried conviction of the truth. The test must reasonably subject his story to an examination of its consistency with the probabilities that surround the currently existing conditions. In short, the real test of the truth of the story of a witness in such a case must be its harmony with the preponderance of the probabilities which a practical and informed person would readily recognize as reasonable in that place and in those conditions. [Emphasis added.]