Very sad to see. Watching the Harry Potter children hit puberty and fuck each other has been the privilege of my life.
In the books there were footnotes for who menstruated and who did not and there was even a table in the appendix to that effect, but everything got glossed over in the films, and I think that's why we're all confused, we needed structure. Not fussed myself, but there could be merit in some sort of witch hunt census run by local authorities. If you held them, menstruators, upside-down would it stay in there or burble out over the top like a lava flow? We don't know, but if it did that would be one way you could find menstruators. You could also try honey dippers, maybe a crowd-funded app, or as a last resort the state could subsidise sanitary products and see who takes them. The nation needs a morale boost and this could help us come together.