You can gauge a pet’s degree of dehydration, or fluid loss, with a simple test.  The first sign of a problem is the skin’s elasticity.  Normally hydrated dogs and cats have extra loose skin at the tops of their heads and bases of their necks-the scruff-that’s easy to grasp.  When the water balance of the body is normal, you can gently pull up the scruff, and when you release it, the skin will spring back immediately to a normal position. 

The more severe the dehydration, though, the slower the skin will react.  With moderate hydration, the skin will go back slowly.  In severe cases, where the skin remains standing up in a ridge off the body even after you’ve released it, first aid and veterinary care should be sought.

Shojai, Amy D. The First Aid Companion For Dogs & Cats. Rodale, Inc., 2001.