There are a few signs to look for to determine whether your baby is getting enough breast milk. First, your baby should have at least 6-8 wet diapers a day and the urine should be pale in color. Your baby should also have 3-6 bowl movements daily, which should look yellow, seedy, and loose (this is after your milk has come in -- the first few bowel movements after birth will look black and like tar). Your baby should be breastfeeding about 8-12 times in a 24-hour period. During each feeding, offer both of your breasts, even if baby is sleepy. After feeding, your breasts should feel lighter and softer. Weight gain is also a sure-fire sign that baby is getting enough nutrition through your breast milk. Your pediatrician monitors weight gain closely in the first few weeks and months of your baby's life.
Breastfeeding is a supply-in-demand process; the more you breastfeed, the more your body will make breast milk. Just because you can't measure the amount of breast milk you are giving to your baby does not mean you can't determine if she is getting adequate amounts of milk. It is very rare for moms not to produce enough milk. If you're still concerned, contact your hospital's lactation consultant or search for your local La Leche League support person.