More often than not, you probably won't catch him in the act. And if you do, it's most likely because he thought you weren't there. I would only start to worry if it seems he's getting dehydrated. Wrinkly, loose-looking skin is a sign of that, although he might look like that if he's shedding/getting ready to shed, so make sure that's not the case before you go panicking and calling vets or anything.