This is one of those things i have never come up with a good solution.
That said, the best "cops" i have seen are just a citizens patrol up in Chicago area. They just make their presence known, talk to all the people living there, and record everything. Crime went way down, and they aren't even carrying guns.
i have seen / interacted with too many thugs that found the blue line gang to be the bestest gang. Many feel like they OWN the city. And are often worse than "City Bosses" of old. (at least you knew you were dealing with the dirty underbelly)
So, how do you police the police?
It is really hard, made harder by the shady people they have to deal with all the time.
But the thing that really gets me is they usually work with said shady people, and rob (asset forfeiture) the good people while terrorizing the average person.
Abolish them.
I don't need police. I might be OK with you having them if you believe you need them as long as they stay in their cop-shop unless someone calls and requests their presence-- like firefighters generally do. But not like the "fire inspectors" who go around molesting people looking for "violations". And as long as no one is forced to fund them without consent. (I do not consent.)
But, really, the world got along without police until the 1840s. They are a recent, defective creation. I don't need or want them in my presence. Ever.