What methods were used in the middle ages to determine guilt or innocence?

People in the Middle Ages did not approach crime forensically, like you see in the CSI shows. In medieval legal proceedings, guilt or innocence was established in three ways, either by direct witness testimony, an oath, or -- until the thirteenth century -- by ordeal. If someone saw a person commit a crime, or was injured or attacked by someone, they would go before a judge and make an accusation. There was generally no presenting of physical "evidence" or "exhibits" like you have in modern criminal proceedings. Both sides, along with any potential witnesses, would be summoned to court and allowed to tell their sides of the story and the accused would either admit guilt and offer to pay compensation to the plaintiff, or would maintain their innocence and swear an oath that they were not guilty. Often times, the accused would have to produce a number of "oath helpers" or members of the community who were willing to put up money or property guaranteeing that the accused person was innocent and would not cause further trouble. If the judge(s) accepted the oath and oath helpers, then the person was free to go unless more information came to light, or they fled the jurisdiction or otherwise reneged on their oath. The status of a person mattered a great deal: poor or unfree people were subject to the whim of their lord's judgment, whereas the wealthy and the noble had more avenues to pursue justice. The death penalty was imposed infrequently, and usually only for very serious crimes such as murder, brigandry, or treason.

In cases where there were no witnesses and no clear way to establish whether the accused was guilty, the court was allowed to use an ordeal, which took different forms over time, but usually involved a dangerous task, the outcome of which was interpreted as a divine sign of who was in the right. One common ordeal was for the defendant to reach into a cauldron of boiling water to retrieve an object. If his arm appeared unscathed, his testimony was affirmed. Nobles and knights would often undergo ordeals by battle in which they personally, or, more commonly, hired professionals, would fight a duel to determine the outcome of a case. The ordeal is often held up as evidence of the irrationality or cruelty of medieval legal thought, but in reality it was a way of forcing two litigating parties to come to an arrangement and settle their dispute. Nobody wanted to end up taking a case to an ordeal because the outcome could not be controlled. In 1215, the Fourth Lateran Council forbade Christians from participating in judicial ordeals.

Despite the popular image of the Middle Ages as violent and cruel, torture as an interrogation tool was not widely used in secular contexts. Torture was, however, employed by the Inquisition to extort confessions from suspected heretics or their sympathizers. The inquisitors appropriated it from Roman law, which allowed for its use in some cases.