The signs of sexual abuse are sometimes tricky to evaluate. Infants and toddlers cannot speak for themselves. Older children and teenagers won't talk out of fear. There are general signs of abuse and signs that are more specific to sexual abuse.
1. The child knows more than what is appropriate for their age about sex and acts out sexually beyond what is normal for their age.
2. The child is behind developmentally or regresses, such as bed-wetting, delayed speech, etc.
3. The child shows fear of a certain place, object, or person.
4. The child harms his or herself.
5. The child has lost interest in activities, friends, etc.
6. The child has pain urinating and defecating. The child is bleeding in the pubic area.
If you notice these symptoms and your gut tells you something is wrong, seek help. Sexual abuse will inhibit a child's growth for the rest of his or her life. The victim may go on to abuse others, not be able to bear children, have significant health problems, suffer from PTSD and depression, develop an eating disorder, self-mutilate, or commit suicide. This is too serious of a problem to idly stand by.
If you suspect you have been abused but don't know it (because your brain blocked the memory), seek help.
This link has more detail about signs of sexual abuse and types of sexual abuse. This link discusses infant sexual abuse.