I've given this question quite a lot of thought, especially since I joined Susans' and discovered the range of aspirations.
Personally, I would say that someone is transgender if they say they are. I can't see anything to gain from winkling out 'fakes'. Since I can't see why anyone would fake this.
If people join Susans' for example, so they can read salacious gossip then they are quickly disappointed.
The claim often made by some in the US, that it's an excuse to sneek into women's toilets is clearly a nonsense since it wouldn't be particularly difficult for such a voyeur to sneek in anyway. Lets face it. A guy, so disposed, could go in, in his normal clothes and hide in a cubicle where he can be disappointed and bored, without having to go to all the bother of declairing himself transgender, getting beaten up a few times, losing his home, family, friends and just about everything else, just so he can be bored and disappointed when he discoverers that girls also fart.
Apologies for making light of that point, but it really seems that ridiculous.