That’s the neat thing: nobody can. It’s incredibly hard to devise a study that can show anything about it. There is no way to get a human without microplastics in them to get a control group, and by this point as far as I know there is no plausible theory to get a specific study.
Everyone kinda suspects that it can’t be good for you, simultaneously there is zero actual evidence that something is ever happening. We don’t know, and that’s very frustrating.