Abuse is measurable in all sorts of ways. The most clear one is having experts take a look at a random sample of users and see if they're being abused. You can back that up with interviews to look for both their take on what's happening and a variety of trauma markers. And there are all sorts of other measures that correlate.

But if there somehow weren't ways to measure it? Then they would have created a product where they couldn't even tell that they were harming people. That right there is something that shouldn't exist.