When the case comes to trial in court, the court will first ascertain the facts on which the plaintiff and the defendant rely.
Facts are ascertained by leading evidence from witnesses, or by producing documents or artefacts. When facts are in dispute, the court must decide what the true facts are.
This is done by weighing the conflicting evidence, taking into account the probabilities, the credibility of the witnesses, inconsistencies in the evidence, and so on.
In the case of jury trials, the jury rather than the judge has responsibility for deciding what facts are proved. Juries are more common in criminal trials than civil trials.