Like the top 1/2" will be lighter in color and weight. You don't want the lower layers to be real wet either, but rather light weight and loose. That is when to water. I like the philodendrum because it is a good water meter. If you let your tank dry to the point that the leaf stem of the philodendrum starts to bend down you really need to water. If you let it go to this point a couple of times you will get a feel for how long it takes it to dry out. Once you have a fair idea you would actually want to water before the philo hits the drooping stage.
The other thing you can do is get a good hygrometer and when the humidity drops below 70 give it a good spritz.
You have a drain on your viv don't you? If so, just give a good rain with a sprinkler of some sort. I use a watering can with a sprinkler head on the spout and the dragons enjoy the rain as much as the plants do. They will rub their heads and bodies on the wet branches, lap up the puddles from the leaves and branches and often this stimulates them to jump into the pool for a good round of swimming, splashing and grooming.
I do have the babies but they are still not ready to go to good homes yet. I have sold out on all but a couple of last years babies and the new ones are only 1 and 3 months old. I will keep you in mind though and will let you know when I consider them to be ready to go.
Marcia