Unless you've bought a new bearded dragon, and you're providing a bowl of fresh water to be available at all times,
the chances of coming across a bearded dragon with dehydration is highly unlikely.
After all, why would he get dehydrated if you've had water available to him?
The most common situation in which you encounter bearded dragons and dehydration is in newly imported
specimens who have been sitting on the cargo hold of a plane without nourishment for several days.
And you're going to buy a captive bred speciment aren't you?!
Still, the signs of dehydration in bearded dragons is that the eyes can appear sunken, the bearded dragon appears listless,
and most commonly and obviously of all, when you pull the skin out gently, it fails to resume it's normal shape for some time.
You know, if you pull out the skin on the backs of your knuckles and let go it springs straight back against your hand.
In cases of dehydration (humans included) when one pulls out the skin it will take some time to "snap" back into place.
If you find this in your bearded dragon it's an almost certain sign of dehydration.
Whilst coming from arid desert regions, short periods of high humidity will help your bearded dragon, so make up a large icecream
tub or similar in which your bearded dragon can move around, yet is almost totally enclosed.
Piece of some holes in the tub with a skewer or a soldering iron so there can be some circulation of air.
Then add a large bowl of fresh water together with lots of moist substrate - damp compost with some vermiculite mixed
in to increase the moisture holding capacity, damp kitchen paper or my favorite which is damp
moss.
Place your bearded dragon inside this tub for several hours at a time then place him back into his normal cage.
Do this several times each day, feeding him a limited diet of high-water foods such as tomato, strawberries and lettuce.
Remember that the body requires and uses up water in order to digest food so only feed limited amounts or you'll be reversing all your hard work.
Test the skin daily, and if within 48 hours you don't see any improvement in your bearded dragon's dehydration, take him to a vet for immediate attention.