There are signals that your baby will give you to let you know if they are tired or hungry. Usually rubbing of their eyes and yawning are good indications that they are tired. Sucking of the fists and rooting are signs that it may be feeding time.
The best way to ascertain that the baby isn't actually hungry is to follow a daily feeding schedule. After a breastfed baby reaches about four months of age, they can go at least three hours between feedings. Generally, a baby who is really hungry will be eager to eat. When the baby goes to asleep without completing a feed, you can assume that they were tired.
Lots of babies aren't hungry according to the feeding schedule. There may be times when the baby will want less food. Other days they may want to be feed more frequently. Babies who haven't had enough food will usually cry between feeding times.