Overfeeding. If you baby is taking more than he/she needs, you will most likely see spitting up, irritability, and weight gain that is faster than expected. Some babies will over feed (if offered), because feeding and sucking are primary comforting methods. This is less likely to happen with breastfeeding. If you are not sure, ask your pediatrician.
Too much weight gain. If baby is gaining weight too fast or you are getting lots of spitting up. Babies should gain about 1 ounce per day or a pound every two weeks. If your baby gains faster than that, then there is a good chance that they are overfed. Overfeeding will also increase spitting up, so with excessive spit up, you are likely over feeding.