Breakin' the Law
Law Enforcement
How did people know a crime had been committed? Same way as now: someone made an accusation. That accusation was typically made to an authorized representative of public authority. There weren't really police yet, though there were officials with police powers. In the English countryside, for example, this was the sheriff. Every community had someone. If nothing else, Joe Peasant would go to the "better sort" in the village and make his claim, and they would then go to the public official.
There was nothing like forensics, though, so officials relied heavily on the testimony of others. They saw X fighting with Y. They saw a man, a stranger, running away and then the fire broke out. That sort of thing. If the accused were known, the sheriff (or his equivalent) was authorized to go to the house of that person and arrest him. He might take the man away to be imprisoned in the local castle or monastery. He might simply tell the fellow to stay put until the next court.
It was no good running away. If you did, you were declared "outlaw"; that is, you were outside the protection of the law. You could be killed on sight with impunity. Your property could be confiscated and not returned. Your family would be left with nothing. Only the desperate ran, and medieval society generally figured only the guilty ran. On the other hand, if you could run beyond the reach of the law, you were free. There was no international police and no rules for extradition. The fugitive could go to another country and be safe. The fugitive could also simply go into the hinterlands and live as a bandit. That's what Robin Hood did.
Sometimes, the guilty man was caught in the act. The village could then rise up against the felon and either apprehend him or drive him out. In England this was called raising "hue and cry", and to do so without cause was itself a crime, sort of like dialing 911 as a joke.
There's one other area of law enforcement worth mentioning, something called "self-help." A good many crimes were dealt with privately, especially in the towns. These included obvious crimes such as assault or burglary, but also included offenses not in any law code, such as disrespect or dishonor. The legal system still recognized the right of a man to redress wrongs done to him, especially when the act was one that did not otherwise affect the rest of the community.
The most notable example of this is the duel.