for the most part, you wont see a cham drink, unless its a montane species, that drinks water all the time.
The eye sunken thing, while technically correct, is not the best way to tell, because some chams will pull their turrets in when stressed or angry.
the Best and most accurate way is to check urates.
if the urate is yellow or orange, the cham is dehydrated. but you have to check fresh ones, because the older urates can be dried out, and can look yellowish, even if they aren't dehydrated.