Instructions
Look at the feathering. A very small bird with few to no feathers or with only downy feathers is a nestling. A young robin whose lower breast is red and whose body is fully feathered including tail feathers is a fledgling. Fledgling robins are often on the ground because they leave the nest and spend a considerable time on the ground while they are learning to fly. While the baby is on the ground it is under the close supervision of its parents who feed it several times every half hour and teach it how to hide from predators.
Observe how it moves. If the young robin seems weak and can barely move, or if it lays in the grass and seems shaky, it is most likely a nestling. If it is hopping around on the ground and fluttering about, it is a fledgling.
Listen to the bird. Nestlings make peeping sounds. If the young bird is chirping loudly, it is probably a fledgling calling to its parents. The chirping is not a sound of distress. It is used to allow the parent birds to locate the young bird in order to feed it and protect it.