What is the test for assessing credibility of a witness?

Assessing the credibility of a witness is a difficult and delicate task, for which there is no predetermined set of rules. Although the courts have attempted at times to offer guidelines, they have not provided an exhaustive list of all the factors that may enter into such a determination, nor have they precisely defined the manner in which to make such an assessment. However, the following passage in Wallace v. Riddell (1926), 21 O.W.N. 202 gives some insight into the process of assessing credibility: …the credibility of a witness in the proper sense does not depend solely upon his honesty in expressing his views. It depends also upon his opportunity for exact observation, his capacity to observe accurately, the firmness of his memory to carry in his mind the facts as observed, his ability to resist the influence, frequently unconscious, of interest to modify his recollection, his ability to reproduce in the witness-box the facts observed, the capacity to express what is in his mind - - all these are to be considered in determining what effect to give to the evidence of any witness.