The short answer is much the same way as you prosecute crimes today.
Take for example the crime of Stalking - No physical injury is done and we can't see mental trauma - how do we prosecute it?
First step is the statements from the Accused and the Accuser - AKA He-said-she-said.
Then we look at all the circumstantial evidence (which, despite what TV Legal dramas would have you believe is good evidence) - for example Person A showed up at Person Bs work 5 days out of the last 2 weeks, despite not having a legitimate reason for being there.
Person A was spotted driving around Person Bs house late at night on several occassions.
Person A sent some rather dubious text messages to Person B etc.
Then you have Means, Motive and Opportunity:
Person A is unemployed but owns a car and has access to funds and is able bodied (they have the means)
Person A attempted unsuccessfully to win the affections of Person B and has been scorned (Motive)
Person A being unemployed has all the time in the world to slink around and make Person B's world a nightmare (Opportunity)
Then, even if a Wound heals quickly - what about Blood splatter? Middle Ages Forensics might not have developed the analysis of Blood Splatter that we have today A la CSI - but I'm fairly certain that if one person said they'd been assaulted in a room and there was blood everywhere in that room, a Middle Ages person would conclude that something had happened.
Now - I will grant you that there are many instances where a crime has been committed, but the evidence is weak and so in-line with English Common Law jurisprudence 'Beyond all reasonable doubt' means that guilty parties go free.
The Trolls will likely be no different - especially since you mentioned the concept of false accusation.