Some key signs are: A baby continues to suck milk for more than 30 minutes or cries shortly after nursing. A baby is not happy and does not sleep well. You do not feel the strain on the mammary glands. A baby excretes hard faeces and is constipated. A baby does not gain weight. Weigh the baby before and after nursing with his clothes on. The difference between the weights shows the amount of milk sucked by the baby. If your breast milk is not sufficient, try to increase the flow without the addition of formula. If this is still not successful, consult your health care professional.