How can I tell if my baby is full? 

The baby's sucking motions will slow down and become more intermittent. The baby will slowly suck the milk and take breaks in between.
The baby's grip on the nipple/bottle will loosen. They lose the energy needed to suck for milk.
The baby's eyes may start to close and they want to sleep.
The baby's cheeks and chin may look rounded and full.
The baby may make digestion sounds during feeding like gurgling and burping.
If the baby spits up the milk, it likely means they are full.
However, babies don't always react the same way. Some babies fill up quickly while others take their time. As you get to know your baby, it will be easier to tell the right time. Also, be careful not to stop feeding before the baby is done. Let them feed a little longer to ensure they get enough.