You know you pass, when you get a bit tipsy, get sick of of constantly wondering if you pass or not and so ask the person you are talking to if they could tell that you were transgender and they refuse to believe you.
I did it with four people through the night as an experiment, after I had been chatting with them for a bit. A few said they would have had no idea and one said I was lying. I think he thought I wasdoing it to attention seek or something. It was quite a boost though and completely counter to what I normally do.
I also got asked back to someones, who changed his mind when I told him I was trans, so another pass, not that I would have gone. It was a good night.