Gray water with hook ups

I have a 1996 TM. Recently purchased and have yet to use it. It is currently being held hostage for repairs at a TM dealership. My question is this. How do you know if your gray water is full when you are hooked up to city water? Please forgive if this is a dumb question. The answer was easy with my tent trailer. Go outside and look at the container that sat on the ground catching the gray water.
I never actually saw my gray water tank during my brief posession of our new toy.
Thanks in advance
Cheri