In most societies that have any glimmering of civilization, a person accused of wrong doing is given at least a nominal chance of proving his innocence. The Romans had a highly sophisticated/comprehensive system of courts and the members of their legal profession were well educated but the Saxons who followed them to rule Britain used rougher methods.
From about the sixth century A.D. to the eleventh the majority of the trials were in the form of cruel physical torture (carrying a piece of red hot iron, stepping berefoot and blindfold across a floor covered with red hot coals or sometimes by a gentler method of oath – swearing.
The accused was ordered to bring to the Saxon authorities, a police officer or a priest could be persuaded to swear on oath or still a number of persons who would say that the accused was of good character and thus innocent. The number of persons who swore depended on the crime. A noble/a landlord or a priest counted for up to half a dozen ordinary peasants. As almost everyone lived in small villages, where almost everyone knew everyone else, and very few would risk telling a lie on oath (the people were mostly religious), the truth was generally told. If the accused could not produce enough oath helpers, he was found guilty and punished.
In the eleventh century the Normans introduced trial by battle in certain cases. The accused and the accuser fought with special weapons until one was dead or surrendered. It was believed that God would know the guilty and give the innocent the power to win. The whole idea became ridiculous when both the parties were allowed to hire champions who would fight on their behalf. It seemed likely whoever could pay the more for a stronger professional fighter stood a good chance of winning and judged innocent. This may sound unfair to us but there is a parallel with a wealthy person today who can hire a costly and brilliant barrister to defend him.
In the early middle ages when England was a land of small villages remote from each other, crime tended to be basic and direct : beating up, theft, sex and murder being the main offences. But as towns and manufacturing and commerce grew, the possibilities for cheating and fraud soared. The whole organisation of society became more complex and opened the door to a world of more sophisticated wickedness. With no regular police force, spies and informers were offered rewards when they brought in criminals.