A good way to know if your baby is getting enough milk in the early days of breastfeeding is to keep track of wet and poopy diapers. But please note that this is not the only way to insure your baby is being properly hydrated and fed. Continuous weight gain and alertness are also key indicators that the baby is getting enough nutrition.
As a general rule, by day 4, a baby should have a minimum of 4 poopy and 4 wet diapers in a 24-hour period. On day 6 and continuing for the first few months a baby should have a minimum of 6 wet diapers and 4 poopy diapers. You should also watch a baby's weight gain. Baby should regain their birth weight by 2 weeks of age, and in the first months the baby will gain approximately 1oz per day. Familiarize yourself with how a dry disposable diaper feels both wet and dry, so you can feel a little more secure with recognizing a wet ultra-absorbent diaper. It is very important to keep in close contact with your baby's healthcare provider to insure all is well.
Sometimes a baby may seem fussy directly after breastfeeding and sometimes people assume that means that the mom does not have enough milk to satisfy the baby. Please be assured that this alone is not an indicator that you don't have enough milk. The baby may need to burp, be re-positioned or just needs to take a break before resuming breastfeeding. Quite often, a breastfed baby will go through periods of "cluster feeding" and you may need to offer the breast again close to when you last fed, if for only a few minutes. While the baby is breastfeeding, you will notice that baby will suck vigorously and then slow down once a "let down" occurs. You will hear swallowing as "an" or exhale or "kuh" sound.