Domestic violence cases are notoriously hard to prosecute because relationships are complicated and an unhealthy relationship is not necessarily an abusive one. Evidence of abuse is also difficult to come by as many of these end up as he-said-she-said cases without physical documentation.
When an officer is dispatched to a domestic disturbance call, there are a variety of signs the officer will be looking for to determine if there is any abuse going on to warrant an arrest and criminal charges. These can include physical signs such as bruises and other injuries as well as broken glass or overturned furniture in the house, indicating an incident.
It's important for defendants to remember, however, that in these cases, there is still little to definitely prove that the defendant is the person who caused the injuries unless there are witness statements. Even if there is enough evidence for the police officer to make an arrest, this doesn't mean that the case will be prosecuted.
Sometimes an upset partner threatens to make a false report for domestic violence as a way of scaring the other person. There are situations where the partner calls the police, and the defendant ends up being arrested, even in the absence of any physical evidence. In these situations, one person may claim that the defendant is emotionally or mentally abusive.
While signs of emotional abuse can help build a case against a defendant if there is also evidence of physical abuse, threatening behaviors or a history of violence, these factors are usually not enough alone to get a domestic violence conviction.
Source: Office on Women's Health, "Violence Against Women," accessed April. 16, 2015