How do I know if my baby is dehydrated?

Poor urine output. There are several ways you can tell if your baby is dehydrated. The most important one is if you notice that your baby's diapers are drier than normal or that he or she has fewer wet diapers. Other signs include dry lips, flat fontanelle if your baby is less than six months old, crying without tears, and decreased activity level.
Fewer wet diapers. Holding on to fluid is the 1st thing we do to avoid or correct dehydration. The earliest sign is a decrease in wet diapers. For an infant, fewer than 6 wet diapers in 24 hours means dehydration is setting in. Disposable diapers make it hard to tell if baby's wet. If you're not sure, use kitchen scale (not bathroom scale). Weigh a dry diaper, then baby's diaper. A wet one will be 1/2 oz or more.