How do I know when a child is dehydrated?

Count diapers. If you are concerned about dehydration in your baby/child who is still in diapers- start counting how many wet diapers they have. Babies will usually have a wet diaper after most feedings- 6-8 a day. If your child is having significantly less or has not urinated in 8 hours please call your pediatrician.
Tired kid with . . . A child (toddler or older) gets dehydrated usually from diarrhea, especially if he is drinking incorrect drinks or amounts, or he simply can't hold anything down. Tiredness & thirst are early signs. Then the mouth gets sticky (instead of wet), the eyes look a little sunken, the urine is little & darker, hands & feet get colder, skin color doesn't look so good, skin gets mushy & dough-like, etc...