I appreciate that question. Unfortunately, it's too difficult to tell. People have tried to think about ways you can trace it.
The best information we have is the paper from Jenna Jambeck et al., trying to say how much is coming from each country, but it doesn't take into account waste that's shipped overseas.
Unfortunately, when it comes to microplastics, it's pretty difficult to know where it comes from. We do better at knowing whether it's tire dust or whether it's coming from washing machines, etc., and identifying the different pathways.