Babies are smart, and sometimes we don’t give them enough credit. Your baby knows the difference between nursing for comfort and for hunger… and he will adjust his suck accordingly.
It isn’t so very different from adults and their feeding habits. Just as adults might eat or drink for different reasons (hunger, thirst, socialization, boredom, etc.), babies can also nurse for different reasons. Though, they primarily nurse when hungry and/or in need of comfort. Bonding is an important part of parenting a baby, and babies quickly learn that breastfeeding is a lovely way to experience that sense of closeness and security.
A baby who is hungry will nurse with a suck, swallow, pause pattern that allows his mouth to fill with milk before he swallows. When your milk is flowing well, he may suck once for every swallow and then breathe, but that suck will have enough of a pause to get a mouthful of milk. If he’s at the breast for a reason other than hunger, he may suck a little and he’ll no doubt get some milk, but not the kind of mouthful that comes from earnest eating.
However, there are some obvious hunger signs all parents should learn. A baby may smack his lips, bring his hands to his mouth, squirm, and even make attempts to find the breast by lurching downward if he’s being held upright. If he doesn’t get what he’s looking for, he will eventually resort to crying. Ideally, mothers will recognize a hunger sign and feed their babies before they cry for food.
Just as there are hunger signs, there are signs of fullness too. He may fall asleep at the breast after a good feed, or slide off the breast with an unmistakable “drunken sailor” look. He’ll relax noticeably as well. Look at his hands—at the beginning of a feed, they are likely clenched in a tiny fist. As he gets full, he’ll relax, starting with his facial muscles and then his shoulders, and finally his hands.
It's important to learn the cues that tell you when your baby is hungry but understand that your baby may seek the breast for comfort, too. There are many non-hunger reasons a baby might want to nurse—a need for comfort, bonding, illness, sleepiness, and so forth. Offering the breast "on request" will ensure that your baby gets the food and the comfort he needs.