We are not talking about cross-dressers. We are talking about transgender individuals. You wouldn't even be able to notice them standing in front of you once their transition is complete.
I don't understand that part. I think there are some concepts that are confusing you. All of the transgender individuals that I have known from before their transition pass as their new gender without any issues. The only way anybody would know if you told them.
Story time:
You know how men always like to boast and compare their masculinity while drinking? Well, one time we were out drinking with friends. A man from my high-school (who was a girl last I saw him) was with us. He is quite manly, has a beard and looks like what you would expect a fashionable young man in his mid twenties to look like.
After a few beer one of the guys turns toward the man in question and, without having a clue that this is a transgender individual and tells him: "We know how it is with you lucky guys who hit the genetic lottery and are so manly. With a beard like that I bet all the girls must be throwing themselves at you. I must be half the man you are." ... My jaw dropped.
It was a ridiculous situation to witness but it shows just how easy it is to be next to someone who has gone through a complete transition without having a clue. If you live in a city, I'm sure that you have called at least one or two transgender individuals by their new pronoun without being aware of it.
I do the same, but also for cis people. It makes life simpler.
For me, I tend to assign gender by sex. I don't want to disrespect myself by saying something I don't believe. On the other hand I don't want to disrespect others either. So for now my approach is to sidestep the issue and refer to all trans people with "they".