We know for sure because in some cases some other evidence were revealed, an example of that would be one of law enforcement officers telling the truth about thier partners either because they feel guilty or for some other personal reasons, but ya it happens all the time.

I am literally telling you that any random policeman can frame you pretty easily, just one pill of drug and you do a year in prison, you have to count on the integrity and honesty of everyone who has the power to frame you, you have no idea how scared people are when a police officer pull them over, you could end up doing a full a year in prison for "consuming drugs" and 5+ for selling it, if said officer has access to a large quantity of drugs they can put that in your car and you spend 5 good years in jail.

"extraordinary claims require extraordinary evidence", since this is someone's life and freedom, if the evidences are even remotely questionable -- they mean exactly nothing.