You prove fault in a wrongful death case by evidence. This can be the evidence of the parties in the form of testimony or direct evidence (like pictures). It can also be in the form of expert opinion. In prior wrongful death cases I have worked on we hire an accident reconstructionist, toxicologist, or medical professional to investigate what happened and form an opinion as to the cause of the death. If the opinion is with a reasonable degree of certainty than a judge will allow the jury to hear the opinion. Your lawyer can help you figure out who to call as a witness to prove your wrongful death case.