How can the police be 'policed'?

I know here in my part of the country, we have a lot of dirtbags that the cops have to deal with. They have confrontations with people several times "every" day. Their jobs are very high stress. If and when they cross the line is usually a decision that is made in a split second and sometimes, they are human and they are wrong. I am not justifying illegal or wrong decisions, I am saying that they are put in serious situations constantly, with really bad people.