Knowing the signs of abuse is the first thing you need to know in order ot prevent child abuse. You cannot report someone suffering from "XYZ" if you don't know what "XYZ" is. Knowing the signs is an important step. Be more observant around you--I grew up with many friends that were abused, but I didn't know if because I lacked knowledge of their lives at home. Pay attention.
Listen. If a child says they are being hit at home, they most likely are not lying. Children most likely wouldn't lie about things like that, and refusing to listen to a child in their time of need makes you just as bad as the abusing parent. Never take their words with a grain of salt, especially if the child insists or keeps telling you they are being abused.
Report it. If you think you know a child that is being abused, report it. If you think the situation is dire, call 911 as soon as possible, and don't take no for an answer. I can't tell you how many times I have called the cops about a child being abused near me, and how many times they have either hung up on me, or never even showed up. It's a life on the line. Keep calling the cops. Call everyday if you have to. You know the truth--now let the police know the truth so that they call handle it.
Talk to the parents. If the parents talk about their children in ways that seems violent or abusive, then keep in touch with those parents. Parents that abuse their children keep a distance from them, or speak of them in horrible ways, so pay attention. A parent's body language with their child can tell you a lot about them as people. So don't push their words or behaviors aside.