I would say, 'Just wanted to clarify something. I'm only qualified to do a basic bikini which means that you leave your underwear on and I wax the crease of your leg. Are you happy with that?'
The above is very tactful and respectful, most transsexuals will be "tucked" for any kind of service like this, it's just the way they are, there is no no to be fearful ....
OR, you can just come straight out and say "can you put the dog under the house luv" with a smile, they will get the idea