The best way to be sure about this is to consult with your physician, and your physician will know by regularly weighing them at visits.
For the most part, you should continue to feed them when they want more and to avoid feeding when they express less interest. Your baby’s instincts should be trusted in this situation, because they will know when they need more food and when they don’t. Signs that they aren’t getting enough to eat include less urine output and lethargy. If you have concerns about this issue, please call your doctor.