This is a great question. Certainly the weight of baby tells you a lot and is the most important indicator of whether the baby is getting enough milk, but there are three other ways as well.
First, if the mother is breast feeding, then the mother will be able to tell how much milk is coming out. If her breast feels softer, then this means that milk has gone out into the baby.
Second, the baby might be an indicator as well. If the baby is satisfied and relaxed, it may be a sign that the baby has eaten enough.
Third, you can check their diapers. After a month or so, babies should have three stools a day. If this is taking place, then this is a good sign. In the first few days of life, this will not be the case. It might just be one wet diaper, because the baby is taking in the mother's colostrum.