If you suspect abuse, it is best to contact Child Protective Services so they can investigate. The child deserves to have the benefit of doubt tip in their favor.
If you are a professional, such as a physician, nurse, social worker or teacher, you have a professional obligation to report suspected abuse (let alone known abuse) and you will be help resonsible if you do not report this to CPS.
IN NEW JERSEY, visit this link: http://www.state.nj.us/dcf/abuse/how/
Here are two snippits of info from the site to help you make your decision:
"In New Jersey, any person having reasonable cause to believe that a child has been subjected to abuse or acts of abuse should immediately report this information to the State Central Registry (SCR). If the child is in immediate danger, call 911 as well as 1-877 NJ ABUSE. A concerned caller does not need proof to report an allegation of child abuse and can make the report anonymously".
"Any person who knowingly fails to report suspected abuse or neglect according to the law or to comply with the provisions of the law is a disorderly person and subject to a fine of up to $1000 or up to six months imprisonment, or both."