What is the test for credibility of a witness?

The following excerpt from the case of Faryna v. Chorny expresses some of the difficulties in establishing sincerity and credibility based solely on the demeanour of witnesses and the necessity to evaluate the reliability of an interested individual’s testimony based upon all of the known evidence: If a trail Judge's finding of credibility is to depend solely on which person he thinks made the better appearance of sincerity in the witness box, we are left with a purely arbitrary finding and justice would then depend upon the best actors in the witness box. On reflection it becomes almost axiomatic that the appearance of telling the truth is but one of the elements that enter into the credibility of the evidence of a witness. Opportunities for knowledge, powers of observation, judgment and memory, ability to describe clearly what he has seen and heard, as well as other factors, combine to produce what is called credibility.... A witness by his manner may create a very unfavourable impression of his truthfulness upon the trial judge, and yet the surrounding circumstances in the case may point decisively to the conclusion that he is actually telling the truth. I am referring to the comparatively infrequent case in which a witness is caught in a clumsy lie. The credibility of interested witnesses, particularly in cases of conflict of evidence, cannot be gauged solely by the test of whether the personal demeanour of the particular witness carried the conviction of truth. The witness must reasonably subject his story to the 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. Only thus can a court satisfactorily appraise the testimony of those shrewd persons adept in the half-lie and of long and successful experience in combining skilful exaggeration with partial suppression of the truth. Again a witness may testify what he sincerely believes to be true, but he may be quite honestly mistaken. For a trial Judge to say “I believe him because I judge him to be telling the truth”, is to come to a conclusion on consideration of only half the problem. In truth it may easily be self-direction of a dangerous kind. The trial Judge ought to go further and say that evidence of the witness he believes is in accordance with the preponderance of probabilities in the case and, if his view is to command confidence, also state his reasons for that conclusion. The law does not clothe the trial Judge with a divine insight into the hearts and minds of witnesses. And a Court of Appeal must be satisfied that the trial Judge’s finding of credibility is based not on one element only to the exclusion of others, but is based on all the elements by which it can be tested in the particular case.