Babies are very vulnerable and find many things very stressful. Obviously, crying or screaming is a major sign of distress and could be caused by a wide range of factors. A parent should know what to look for when a baby is clearly crying in distress, because it could simply be that she is tired or it could be far more serious, such as a spider bite or a serious illness. Poor appetite can be another sign, as can irregular breathing.
Young children are beginning to be a bit more articulate, yet they still have ways of showing distress that might not be immediately apparent. Wetting the bed is a clear sign of some sort of abnormality in their lives. Weight gain, irrational fears, withdrawal and isolation, aggressive behavior, escapism into TV or video games -- all can be signs of deeper problems that need to be addressed.
Adolescents can display many of the signs of young children, such as withdrawal, aggression and escapism. Drug and alcohol abuse could begin at this stage and is usually a fairly good warning sign that something is troubling them. Adolescence is a time of transformation to adulthood and can be bewildering and stressful. Other signs include eating disorders, sexual promiscuity and abnormal or inappropriate sexual behavior. Unsavory company and an attraction to gangs are warning signs as well.
Older children such as your child away at college might be highly articulate but will not express their feelings due to perceived feelings of weakness or embarrassment, so being alert to signs and signals is still important even at this age. For example, a decline in their normal academic performance could indicate problems. A withdrawal of contact or conversely an increased reliance on contact can also be a display of underlying distress. Talk of suicide, bizarre behavior or speech and weight gain or loss can all be warning signs as well.