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...

I don't know how I'd make it work exactly, just in principle. I envision a group of people who have enforcement powers over the police, but nobody else. But in order to have enforcement power, they need to be immunized against retaliation from the police. Part of why I think they should have no power at all over anyone other than the police is to reduce the damage potential.

Clearly there are details that a policy expert would have to work out, I'm just a rando on the 'net philosophizing about how we as a society can stop the slide into a police state and preserve a healthy democracy.

Maintaining a liberal democracy seems hard, like it's an unstable creation that can only be sustained through deliberate effort.

In NL, the authority watching the police (Marechaussee, [1]) is organizationally a branch of the military, next to the army, navy and air force subdivisions. I guess it would be similar to having the National Guard be the police watchdog in the US.

The Marechaussee are also involved with other police duties such as fighting organized crime and riot control. NL police doesn't have SWAT teams like in the US, they would be part of the military hierarchy, not the police hierarchy.

[1] https://en.wikipedia.org/wiki/Royal_Marechaussee

I assume this meant only that their org chart didn’t eventually merge with the police’s. Kind of like SREs at google.