If I see any of my chameleons drink, I am worried something is wrong! The only time I see them drink is after the females have laid their eggs, otherwise I never see them drink. I have an automatic misting system and run it a lot.
As someone posted, dehydration will show up with orange urates (the white part). Sometimes the tip of the urate will be orange, but that is okay.
Usually before the urates become orange, dehydration will show up in the eyes and the skin. Everything sort of deflates--you notice it around the hips/back/base of tail. They just suddenly look skinnier and their eyes might appear to be sunken in.