Hi, I hope I can help you.
If a child is being sexually abused, obviously, the poor child will not speak up. If you are close to a child who you personally think is being sexually abused, you have to tell the child that they are not alone and that they can tell you anything. But please do NOT jump to conclusions and do NOT say anything unless you are very sure about it.
A child who is being sexually abused may act jumpy or a bit frightened. They might like to spend time alone, a sign of depresion. They might not to things which an ordinary child does, and they might act or sound weird.
Please be sure about what you do and your actions, because remember, this is just advice. If you want to seek more proffessional advice, get help.
Good Luck :)
PS: Do NOT jump to conclusions