How do we police those who make racist and sexist threats? Simple. The same way we police african-american kids. Shoot them in the back, for starters, we as a nation and society and government and“peace-keepers” seem to have no issue with that approach.
But seriously, lets start making some False Positive errors, after all, we accept those kinds errors for pre-emptive strikes around the world and for justifying official police violence against racial minorities. So a few “innocent” threateners might get caught “unfairly”, but it will save tens of thousands of true potential victims from the actual racist and sexist violence committed by the modeled “lookalikes” of the “mere threateners with no intention of actually committing violence” you seem so desperate to protect.