It is easy to prove otherwise. Not sure why the downvote. Calm down.

Sans dashcam how do you prove whether someone backed into a vehicle, or drove into the back of a vehicle?

I'm talking about slowish speeds here. Obviously if someone is rear ended at 50mph the culprit is obvious because no car can reverse that fast

That's where "presumed liability" enters the picture. Sans dashcam, the cop can gather other kinds of traditional evidence such as witness accounts, but otherwise the person in back is presumed to be at fault.

Sure, but I was specifically asking the poster above me, because they said "It is easy to prove otherwise. Not sure why the downvote. Calm down."