"So, how do you police the police?"

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.