All of this stuff about what bathroom Transgender people are entitled to use has got to be one of the dumbest things that I've seen people get all up in a huff about.
There are really two distinct levels to this conversation.
One level is the question of the individual you used to know as John who now wants to be recognized as Joan. One may run into this sort of situation at a school or in the work environment.
Then there's the Transgender individual that you don't know. You might run into this situation out in public such as in a department store.
Let's start with the second one because I think that's the easier of the two. In this case you'd probably never be able to tell the difference. Transgenders tend to look like their preferred gender. I defy the average person to recognize a Transgender if it wasn't someone they were familiar with.
Besides, in the ladies room everyone is in stalls. In the men's room a Transgender isn't going to use the urinal so they'll be in a stall as well. So, unless you're into peeking into bathroom stalls, you would never know the difference.
The first case is a bit different. I can sort of understand the possible discomfort associated with accepting that John is now Joan, except not really because the plumbing hasn't changed any. But again the stalls in use idea applies here as well.
A locker room is a bit more complicated. Other than having some sort of screened off area I don't see any particularly good idea here.
The best I can come up with is all Transgenders use a screened off area in the boy's locker room. Guys tend to not give a crap anyway and there's less parental protection foam around boys.
Of course we could just all recognize that individuals should use the facilities of their gender identity and get over it.
Do I have any direct experience? I have two actually but one doesn't really count because it didn't come into the open until years after I knew the individual involved.
The other involved a guy/girl that sort of looked like he was in transition to she. He/she worked for the same place I did. Everyone pretty much ignored it. I have no idea which rest room he/she used nor did anyone seem to be concerned about it.