My skink only goes on one side of his/her enclosure too, on the cool side. Once the little guys become active for the day, there's a good chance they'll think about going to the toilet soon. However, they will go when they want to, whether they're outside on the grass, sitting on you, or just after being fed. The second one isn't pleasant, but try not to freak out if it happens. The last one isn't a problem unless you feed them on your bed or on the carpet, I was lucky in that I always feed my skink, Gizmo, on an old shirt.

If you want to know where she goes in her enclosure, look for urates if the substrate you use is dark, or poop if the substrate is light. Alternatively, walk into the room her enclosure is in at the same time that she is going to the toilet to know which part of the enclosure she uses as an outhouse. She probably won't poop or pee where she sleeps, so that's one area you could cross off the list of potential pooping areas.
