In general, a hungry baby will let you know. Watch for rooting (an open mouth that seems to be constantly searching for a breast), sucking (on fingers, on your shoulder, at the air), and fussing or crying.
Crying is considered a "late" sign of hunger, so ideally, you'll learn baby's earlier cues over time. But if she does start to wail and you aren't positive it's a "hunger cry," use the process of elimination: If nothing seems to soothe her besides nursing, there's a pretty good chance she needs to eat. (Yep, even if she just ate an hour ago.)