the easiest way i to know, check her eyes, if sunken in, she isnt drinking, if they are not... she is... you can also tell by the texture of her skin, or rather wrinkles and folds in her skin. you can also try to keep an eye on her, visual confirmation is good. I have a cheap shower head that has a very nice mist setting... if you are unsure, you can place her enclosure in the shower, making sure that you check the temperature of the water that will be "dousing" her, and let it "rain" on her for 20-30 minutes. keep an eye on her. you dont want to drown her. i do this about 1 time a week as a precautionary measure.
The easiest and most obvious way is to check the faeces. There is a brown section and a white section. The white section is the urate part. If this is yellowish there is slight dehydration. If it is orange then the chameleon is definitely not drinking properly.
If it's pure white or just a little bit beige you can be happy that the cham is drinking fine.