Talk with some of the "The Police are Oppressors!" groups and you get a litany of police wrongs.
And there's a lot to be worried about from no-knock raids, to random TSA checkpoints, to SWAT/Ninja/militarization to the repeal of the Peelian Principles.
There's also those who are against the police for breaking up their blockades, going against their squatting, and screaming about how they tend to arrest criminals.
The problem is often the methods the Police use on "hippie womping" can be quite wrong.
Here's a pretty good litmus test I've found. Ask them about "Only Ones". If they think the State (police, military, ect) are the only ones that should have arms, then they're not against a disparity of force among the Organs of the State. No they just want the State at their beck and call.
Similarly ask them about May Issue. If someone is perfectly fine with the Police having discretionary powers on who can and cannot bear or own a gun. Well... you have to wonder what other fundamental rights they'd be okay with the Police having control over.
Remember some people that complain about Police Brutality do so not because of the brutality but because of the Policeman's choice in target.