I got this information off the web:

"You can tell if your cat is dehydrated by doing the 'pinch' test. Take a bit of skin on the cats back and pinch it, let go and see how long it takes for the skin to retract back to its normal spot. The skin should be extremely elastic and immediately bounce back, if it takes more than a second or two your cat is probably dehydrated."

Additonal information found here : dehyrdration

I would be calling your vet ASAP.