One difficulty in recognizing abuse and neglect is that many of the signs are ambiguous. A child may have gotten a bruise from being beaten by an adult or by having run into a soccer goal. Moreover, children respond to maltreatment in different ways – some show no signs of abuse, while others have dramatic changes in behavior.
Familiarity with the child helps in interpreting any of the signs you observe. A sudden change in behavior, sustained over a period of time, suggests that the child needs assistance. The reasons may or may not have to do with abuse.