I live in Singapore, but in my work I made a lot of trips to Thailand with guys from the US. They always wanted to "go see the trannies", so we went. These places had bar girls who are cis also. It wasn't long before one of them was sitting in his lap. Then he'd spot someone and say, "That ones a tranny. It's easy to tell". To which she would respond, "Oh!, you so clever. They don't fool you". Of course, the women on his lap and half the girls in the bar were transwomen.
Sometimes, the couple would disappear and I'd see him the next morning, totally smitten and talking about how is going to move to Thailand and spend the rest of his life with her.
I know guys who actually did and they have been together for decades and he still doesn't know. My attitude is "if you can't tell, it don't matter".
Interesting enough, the guy who moved to Thailand doesn't understand why she doesn't want to marry him. My understanding is that in doing so the fact she is trans would be discovered. That's why.
This is not the case in Singapore, which everyone thinks is so conservative. Once you get a sex change here, your ID is changed and you are 100% a women in all legal senses of the term. You'd need to hire a private investigator to find out and who would do that if they don't suspect anything. I'd say most of the trans-community here really frown upon getting married without telling, but it happens and no one is going to out anybody for it. The UK law seems so archaic by comparison.