Around the 4th day of life, your baby should have 6 soaking wet diapers in a 24-hour period. If, after the 4th day, you see any pink or red in the urine, that would be cause to think that baby wasn't getting enough to drink. Also watch the baby while you are nursing. Pay attention to the way that he suckles at the breast. You can also occasionally hear swallowing, which is nice, but some babies swallow so quietly that you can't hear them.
Weight gain is, of course, another indication of getting enough to eat.
Of course, many growth charts are based on formula-fed babies, so you need to keep that in mind. Usually, as long as baby is steadily gaining weight at each checkup, there is nothing to worry about. There are also signs of dehydration that you can watch for. Dehydration is very serious, so if you suspect it, go to the doctor's immediately. The best way to ensure success is to just devote all of your time and energy to nursing.