What is the legal test for adding a defendant to a claim where the limitation period has not yet expired?

Master Dash in Wong v. Adler, at para. 45, outlines the approach a judge or master should take on a motion to add a defendant where the plaintiff submits the limitation period has not yet expired because she or he did not know of, and could not with due diligence, discover the existence of that defendant: In my view, as is clearly implied in Zapfe, the motions court must examine the evidentiary record before it to determine if there is an issue of fact or of credibility on the discoverability allegation, which is a constituent element of the claim. If the court determines that there is such issue, the defendant should be added with leave to plead a limitations defence. If there is no such issue, as for example where the evidence before the motions court clearly indicates that the name of the tortfeasor and the essential facts that make up the cause of action against such tortfeasor, were actually known to the plaintiff [page477] or her solicitor more than two years before the motion to amend, the motion should be refused. If the issue is due diligence rather than actual knowledge, this is much more likely to involve issues of credibility requiring a trial or summary judgment motion, provided of course that the plaintiff gives a reasonable explanation on proper evidence as to why such information was not obtainable with due diligence. That is not to say that such motion could never be denied if the evidence is clear and uncontradicted that the plaintiff could have obtained the requisite information with due diligence such that there is no issue of fact or credibility.