How do I know if my baby is really full when I give formula?

It is simple when breastfeeding: A baby generally drinks only as much as it actually needs.

If you give your baby infant formula as well (dual feeding), you can tell from how he/she acts. If he/she drinks the whole bottle up, he/she might not had enough. 

Important:

Prepare enough formula so that some is left over and the baby can decide when to stop.