You often don't see them drink but the colour of the urates when they poo will tell you. They should be white.
Also if eyes appear at all sunken it's a sign of dehydration.
You say you have a 'waterfall'. If you mean a shop bought thing with a little electric pump, these are frowned upon as they are a breeding ground for bacteria. If you must use one, you need to be meticulous in keeping it clean EVERY day.
You don't mention any misting, only the waterfall. It's really important to get your husbandry right BEFORE problems get a chance to happen.
Get rid of the waterfall, it has killed many chameleons, unless you clean it every other day, get rid of it, there are many watering options listed on this forum, if you care about your cham read up on it, they also don't like to be handled, maybe some could be trained to be tolerant sometimes, in nature they are very territorial, rendering them solitary lives, until breeding time