Estimates of future earnings usually require expert testimony involving the deceased person's future earning capacity.
After O.J. Simpson was found not guilty in the criminal case, the families of Nicole Brown Simpson and Ron Goldman sued him in the state court in California for wrongfully causing their deaths. Civil lawsuits for wrongful death are tried for money damages, not to put the defendant in prison. Civil cases are brought in the name of individuals, not in the name of the State.
The burden of proof in criminal and civil cases is different. In criminal cases, the standard is "proof beyond a reasonable doubt." There is also a presumption of innocence that stays with the defendant until and unless the jury returns a guilty verdict. In civil cases, the standard of proof is "by a preponderance of the evidence," which essentially means "more likely than not," or put another way, proof by 51% or more.
Almost every state has enacted a statute permitting a lawsuit to be brought by the relatives of a person who died as a result of a wrongful act.