Rather than "police will be assumed to be lying", I think the appropriate approach is "compare the stories told by both sides for plausibility, considering what

Rather than "police will be assumed to be lying", I think the appropriate approach is "compare the stories told by both sides for plausibility, considering what you know about the world and what you can tell about the character of those involved". So when the police officer says "And then he shoved me off the bridge into the stream. I don't have a recording because the camera shorted out." you find that plausible. When the citizen says "After I filed the complaint, officer Briggs just 'happened' to stop my car a block from my home... and that was the one day when there was an 'accident' with his recording device." that's fairly persuasive.