The easiest visible indictors would be, lack of urates in the bedding, constipation, shedding problems (as Dutchy mentioned), and the fork of the tongue sticking together.
Some snakes, particularly those from more arid areas, can get most of their hydration from their food source. When water is available though, they will readily drink. chaps76, yours will be drinking, and likely quite often, but snakes are often shy and drinking is a vulnerable pastime (you can't watch for danger or tongue flick to smell it when drinking), you just won't be awake/around when it sneaks it's drinks in.
You have a closet drinker.
