This tip was shared with me at a course I attended called Breastfeeding Management, taught by Hellen Sullivan, RN, IBCLC.
When newborns are hungry, their hands are fisted and drawn tight against their chest. However, during a feeding and once their tummies begin to fill up, their hands open, their fingers soften, and their arms lie lower across their body.
The American Pregnancy Association also confirms this easy indication of satiety/satisfaction from newborns at the end of a feeding:
The baby looks relaxed, “falls” off the breast, has open hands, and/or falls asleep. Source
In my house we called this ‘milk drunk.’ :)