Just who, exactly, are you expecting to say "prove it", in this context?
The trans person will merely trot out some concept of "biologically female" that invokes the hormone concentrations they have achieved by taking pills, or else, like one I saw on Twitter, say that since he is female (self-defined) and he is also biological, he is a biological female.
Nevertheless, you need someone in a position to say the "prove it" part, and to enforce that, and to tell the trans person that he can't have what he wants unless the results are satisfactory. Satisfactory to whom, anyway? What is the point of all this?