What is the test for 'discovery' in a personal injury action?

While this question of knowledge can be a factual one, "when (the) knowledge had by or imputed to plaintiff is such as to compel the conclusion that a prudent man would have suspected the fraud, the court may determine as a matter of law that there has been 'discovery.' " Helfer v. Hubert, 208 Cal.App.2d 22, 24 Cal.Rptr. 900, 902 (1962).