HRT makes their hormone levels similar to cis-women's right? So that wouldn't work.
One could look at their privates, but obviously that's very invasive and also kind of gross if they're butchered down there.
What about blood tests? Would it take weeks at a specialized lab to figure out whether they are XX or XY? Or is it something where we could take a drop of blood and find out in 30 seconds?
What other ways are there? What would be the best way to determine birth sex without the person knowing, could you steal a hair sample to test or something like that?