It's unbelievable how many people claim it's not always possible to tell the difference between a man and a woman without checking inside their underwear.
There are so many signs that a man is a man and a woman is a woman apart from the obvious and many are impossible to fake.
I know a number of trans men and women and none of them pass for the sex they want us to think they are, despite some of them having hormone treatment and surgery. Therefore, I think it's unlikely that I'm being fooled by strangers.
This morning, I was in Sainsburys where I saw a woman who had gone to great lengths to look like a man. She had done as good a job as I've seen and was the closest I have seen to a trans person who "passes".
She was tall (probably not far off six foot), flat-chested, bearded, had short hair and was wearing masculine clothing. It was clearly her intention that people should think she is a man, and from the front it was easy to see why some people - at first glance - might be fooled.
However, there were some clear indications that she is a woman. Despite the beard, the feminine contours of her face were evident. She was slightly overweight and - apart from the flat chest, presumably achieved by bi-lateral mastectomy - her fat distribution was clearly feminine.
Any doubt that might have remained was instantly dispelled as she turned and walked away. The shape of her back and hips, defined waist and gait gave her away. In fact if I had seen her from the back before I saw her from the front, it wouldn't even have occurred to me that she was trying to look like a man.
Despite all the trouble she has gone to to look like a man, I would have absolutely no problem sharing a female-only space with her. I believe most, if not all women would realise she was a woman, even if they had to look twice.