There are several of different options. The most obvious is an independent line of authority delegated from the state's legislature or governor. This is where

There are several of different options. The most obvious is an independent line of authority delegated from the state's legislature or governor. This is where the police get THEIR authority. Other options include close supervision by the courts, private rights of action (i.e., citizen lawsuits), strong FOIA-like transparency rules, etc...