Parents are smart.
They listen to their child talk and know how he or she communicates. They also listen to his or her playmates who are about the same age and may even remember what older brothers and sisters did at the same age. Then the parents mentally compare their child's performance with the performance of these other children. What results is an impression of whether or not their child is developing speech and language at a normal rate. If parents think that development is slow, they may check out their impression with other parents, relatives, or their pediatrician.