How does the body measure dehydration?
From Wikipedia entry on thirst:


  In the mammalian brain, the posterior surface of the hypothalamus
  forms the front wall of the third ventricle (a cerebrospinal
  fluid-filled cavity) and clusters of cells (osmoreceptors) on this
  surface, notably in the organum vasculosum of the lamina terminalis
  (OVLT) and subfornical organ (SFO), signal this cellular dehydration
  to other parts of the brain, and thirst is experienced. Destruction of
  this part of the hypothalamus in humans and other animals results in
  partial or total loss of desire to drink even with extremely high salt
  concentration in the extracellular fluids.1


The entry on osmoreceptor says in part:


  When the osmotic pressure of blood changes (i.e. it is more or less
  dilute), water diffusion into and out of the osmoreceptor cells
  changes. That is, they expand when the blood plasma is more dilute and
  contract with higher concentration.


It also describes how the kidney measures chlorine anion flow through some nephrons, which triggers a cascade of messenger molecules resulting in increased blood levels of the hormone angiotensin, which also results in thirst messages originating in the hypothalamus.  

1Derek A. Denton (8 June 2006). The primordial emotions: the dawning of consciousness. Oxford University Press. pp. 118–19. ISBN 978-0-19-920314-7.

2Walter F., PhD. Boron (2005). Medical Physiology: A Cellular And Molecular Approach. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 872