I know that if see a child being abused, I should report it, but what if I read something someone has written that gives me a feeling that they have sexually abused a child? Is that enough to report?
If anything sparks a reasonable suspicion that a child is being abused, no matter how it is obtained, you should report what you know to child protection authorities in your state. They will weigh the probative value of what you reported.
Recently, a New York teachers' aide was charged with sexual abuse of a child. His victim was not a student, but a child outside of the school. The main piece of evidence against him was a notebook in which he recorded the abuse, some images of children, and nothing else, although it is believed he was committing the crime for years. https://www.syracuse.com/crime/2022/08/cny-teachers-aide-charged-with-sexually-abusing-child-over-several-years.html
Child sexual abuse is a crime committed in secret. Eye-witnesses are rare. Victims do not discuss their abuse for many reasons and offenders rarely admit their guilt.
For the most part, physical signs of sexual abuse are covered by clothing and the emotional signs are often mistaken for something else. So, you have to report your reasonable suspicions of abuse as you receive the information, no matter how it was presented to you, and let child protective services evaluate its value.