My question, at what point can anyone just say "fuck it, I am feeling transgender today, maybe I should hang around the women's bathroom and take some selfies?".
As far as I understand it, at no point. You may as well be asking at what point can anyone say "fuck, it, I am feeling like a cop today, maybe I should pull some people over and write them tickets." Sure, you can do that, but don't be surprised when you end up under arrest.
Based on what I've read, I think that a diagnosis of gender dysphoria is needed, which isn't easy to get and requires that the person undergo extensive screenings with psychologists and or psychiatrists, as well as other specialists, over a long period of time. So I don't think you have to worry TA.