Generally speaking, fault is determined in several different stages. First is during the insurance claim process. During the insurance claim process, an insurance adjuster for both sides will be investigating the accident and determining who is at fault.
They may have an investigator take a look at the case. They may interview witnesses, and then once they have all the information available to them, they’ll make a determination of who’s at fault or how fault is to be broken down.
If the case needs to go to court, if you can’t settle with the insurance company, then fault will be determined either by a judge or jury who will again listen to all the witnesses, review all the evidence, and determine who’s responsible for causing the accident.