It practically has to handed to you on a silver platter.
Like the cop literally yelling slurs while it's happening.
I think it's probably inherently known in many hoods though. Look at Ferguson, probably a racial thing there with the white force constatnly having to police certain black areas. It just gets built in.
But "hands up don't shoot" Michael Brown was anything but the case that displayed it.
Without the proof, and just some vague numbers, it's a tough one for sure. We know it's there, more than we want to admit I'm sure.