Video is obvious, but assuming you use a modern phone, you might as well add other sensors like accelerometer data, and extra channels like all WhatsApp conversations.

> I'm going to assume you mean a video recording, which is the only way to be absolutely certain about what occurred when you have exactly two witnesses who also happen to be the accuser and the accused.

Alas, even videos can be ambiguous. Eg picture some BDSM play. There can be lots of context required to make a decision.