I'm an ex-cop from a department in Virginia. When it comes to domestic violence, you talk to both parties, at least if you can. Sometimes, the aggressor leaves before we show up. I had not been to one, but sometimes the victim in unconscious or dead on arrival. You get your probable cause from the totality of the circumstances. Sign of injury is one. It's tougher to figure out if both people were hitting each other, both have injuries and both are lying to the cops? Who's bigger? Who started it? So, I went to multiple domestic calls where the woman is the aggressor and she's the one who gets arrested. I've been to calls where both parties were yelling but no one hit each other and no one gets arrested.
In the cases where it's hard to tell but someone gets arrested, it all gets figured out in court.