There is no internet (let alone social media or body cams) during that era, since Jim Crow in America was still being enforced towards black people at that time, but the civil rights movement is gaining traction (since Martin Luther King was alive). I mean, cop violence has been around for a long time before body cams were invented. How did people hear first hand encounters from those victimized back then without reliance on social media?
Often, it just wasn’t well publicised, which is why police - and people friendly with the police - got away with so much more shit than they could now. If there weren’t a ton of unfriendly witnesses, with no video they could just lie. If there was, the papers had the option of covering it, but the average journalist was friendly with the police. They might just make a gentleman’s agreement that whatever incident was either a regrettable mistake, not worth upsetting the public about, or even that it was illegal but good. Things had to get pretty crazy bad before it got reported.
If you were a black person, you and people you know just very directly felt it. The police could harass or beat you whenever they wanted, which was often enough. And that was on top of the more open racist policies that existed.
Do consider, as well, that informal violence was more normised. Now, if you punch somebody you probably get charged with assault. Then, brawls happened fairly often, and as long as it stayed fisticuffs, it tended to be considered a personal matter. Which meant getting roughed up by the police probably wouldn’t be considered brutality at all until deadly weapons come out, or it’s an extended beating while you’re restrained.