You can tell if a cat is dehydrated by pulling the skin behind their head at base of their neck (just like a Mother cat would pick them up). If the cat's skin stays up when you pull on it, they are getting dehydrated. Get them to a vet immediately. Once a cat becomes dehydrated, it is crucial you get them to the vet. It doesn't take long for them to start losing weight and other health problems quickly develop if they're not getting the appropriate amount of fluids in their system.