When assessing the truth of the testimony of any interested witness, what is the test for determining the truth?

When assessing the truthfulness of the testimony of any interested witness, I am guided by the words articulated many years ago in Faryna v. Chorny, 1951 CanLII 252 (BC CA), [1952] 2 D.L.R. 354 (B.C.C.A.), at 357: 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.