Perhaps the earliest sign of speech and language development is an infant's crying. This serves multiple roles such as indicating need or displeasure. The infant, often, after several months, elicits different cries depending on the reaction desired. Infants' hearing abilities can be gauged by seeing whether they respond to loud noises and the soft voice of a parent or other caretaker. The soft voice and use of language -- such as in singing or the like -- can have a significant calming effect on the child. By the time they are three months, the child frequently can express pleasure through smiles and cooing.
There are a number of developmental identifiers that the NIDCD (National Institute on Deafness and Other Communication Disorders) suggests you look for within three months to a year of age: the child's attention is caught by the sounds of a toy, household noise or by music playing in the background. The child may also noticeably react to how you manipulate or change the texture of your voice when you are speaking to him. Children in this age range will also develop some simple sounds or attempt to mimic sounds directed toward them. They may try to mold expression through these sounds, gurgles and movements of their arms. Simple words such as dada and mama can be achieved by the one year mark as well.
Between one and two, children typically become involved with story books, songs, nursery rhymes and cartoon or children's programs. They can identify things like animals and such through pointing, NIDCD states that these kids often express simple commands or needs like "more food/ juice" and rely on consonants to form many vocalizations. New words appear all the time and, by three years or so, the family understands a great portion of what the child says. Speech is refined around k,f,t,d, and n sounds. By four years old, they should be able to hear your calls from a room or two away from them, narrate a day's activities and effectively form questions. Sentences, by this point, can exceed four words and lack the syllable repetition often characteristic of earlier stages.
NICDC suggests that if you truly feel there are markers of development missing in your child's behavior, hold a consultation with the child's pediatrician. She may alleviate any anxiety right away by telling you not to worry. Not all children are the same and some exhibit traits earlier or a little later than others. However, if the physician does feel there are grounds for further investigation, he will likely refer you to a speech pathologist. In some circumstances, your child's speech and communication development can be encouraged through stimulation exercises before any form of therapy or other treatment is necessary.