The point is, that you can't.
You therefore have to assume that any man who walks into a woman's space has one.
It's totally unworkable, and needs to mean that no men in women's single sex spaces, at all, ever, regardless of surgery, paperwork or anything else, and third spaces provided for those who do not wish to use the provision of their sex.
It's also irrelevant, as a man having a piece of paper makes no difference to the experience of the woman needing to use that space, and makes no difference to the women who cannot use a mixed sex space - which the man has made it by walking in. Which means that he meets his needs at the expense of hers, and excludes her from all provision so that he may take his preferred choice from all the provisions.
And it becomes an action of male dominance, sexism, exclusion and harm, and excludes some women from all provisions and equality of access to society.
Mr Starmer does not want to talk about those women, at least not in front of the electorate before an election. But considering he is deeply concerned about the indignity to a man of a paperwork procedure but not at all about the indignity of a woman having to undress and pee beside any random man who wishes to be present, and seems to feel that the woman having to do this somehow helps and makes it up to distressed men and women should just accept this, it's not difficult to imagine why this might be.