I can answer that one, at least as to my state. In my state, all reports are initially taken by a single centralized office - even reports made by CPS workers if a new allegation pops up. This office then decides whether each allegation is report worthy, or whether a notation just needs to be made in the file. Essentially, an allegation is going to be worthy of an actual report when the allegation would require an investigation if it were true. Example 1: A call comes in alleging that a parent is not allowing their child to drink soda. We don't know whether or not this is true, but even if it were, it's not abuse or neglect so no investigation. Example 2: A person calls and claims that a child is never fed, and is being forced to live up in a tree as punishment. If this were true, it would be abuse/neglect of a child, therefore, investigation. An investigator goes out and either substantiated or unsubstantiates the allegation. If it's unsubstantiated, that's that. There will be record that a call was made but that the child was not found to be at risk of harm, so CPS thanks you for your time. No one sincerely believes all of the allegations that come in because this job opens you up to a world of crazy.
Most people with CPS aren't total idiots, nor are they sociopaths with God complexes. In fact, I knew quite a few with master's degrees and JDs. My state has a minimum degree requirement and pay is a teacher's salary with minimal raises. There are no bonuses, they even removed retention bonuses. No one is there because the job is glorious or fun. It's not. You show up every day because someone needs to. There's a reason stories about kids being removed for playing outside hit the news - because that's not the every day. What the public tends to hear is a lot of "CPS kidnapped my kids for no reason," while for confidentiality reasons, CPS can't respond. The reality is that no one wants to tell their cousin, or their neighbor, or the media, that they actually shoot up with the baby in the same room and CPS was called because their baby cried for 6 hours straight and they were found passed out and unresponsive. Or that they got angry and dipped their child in a scalding bath. I absolutely believe that CPS screws it up sometimes, but very rarely does anyone truly hear what's going on.
Sorry, that ended up far longer than intended.
Tldr: If a report comes in from someone just looking to screw up your day, it likely won't lead to anything. If the report alleges something that would abuse or neglect if true, then an investigator checks in with all parties and will unsubstantiate the claim when they find no risk of harm and that's the end of it.