Juvenile hummingbirds look just like the females before the young males develop their red gorget feathers. But if you spend time watching, you can identify some of the juveniles by behavior.
Tentative and slower in flight initially, juveniles investigate everything, including you and every colorful flower. In only a few days, they show the aggressive temperament that hummingbirds are so noted for. The chasing and conflict we see in our gardens is practice for survival on wintering grounds in Central America where they will compete for nectar and insects with other hummingbird species.