The secret to knowing is paying attention. That can be a challenge if you suspect something or suspect nothing. Look for changes in appearance and behavior. Bust denial at every turn and don't write curious changes or oddities off to age or peers. Children who are being abused may act out their abuse on a peer or sibling since the conscious awareness of abusive events are too overwhelming, and it is common to coerce the abused child into keeping secrets.
Inquire at school about any changes in behavior, attention span, performance, and relations with peers. Shame can lead to withdrawal and acting out or in--that is, inflicting punishment or pain on oneself.
All children cope differently but there are some subtleties that will show up in their behavior and demeanor. Hope this is helpful.