As far as I can tell, Brave, yes. But that accords with self ID etc. If being 'trans' is about someone's inner feelings, then there's no objective, outward way of telling someone is trans or not, of measuring, or knowing, unless they tell you.
If we operate on self ID (as the NHS does, as the MoJ does, as the police do) then we can ONLY tell if someone is trans or any other identity when they tell us.
There is literally no outward sign or way of knowing somebody's 'gender identity'. I would agree with this, actually. We have no way of knowing how somebody feels.
Where Gregor and I would differ is that I would say that in most cases people's sex is 1. easily observable and 2. important and relevant in many instances (3. and often more important in terms of social contracts than the feeling inside someone's head about how they relate to sex stereotypes).
This is why males are now asked if they could be pregnant when presenting for medical procedures.