The first sign of dehydration is the loss of the skin’s elasticity.  Cats and dogs have extra loose skin on the tops of their heads and at the bases of their necks – the scruff – that is easy to grab.  When you grab that skin and then release it, it should spring back into a normal position immediately.  The skin at the top of the head is more likely to show this effect, so you may want to test there first.

If your pet is dehydrated, that skin will not bounce back immediately.  With moderate dehydration, the skin will go back slowly, and in severe cases, the skin will remain standing up in a ridge even after you have released it.  In either case, immediate first-aid and veterinary care are required.