How do you know if you have been sexually abused?

Contrary to common legend, most people who have actually been abused have memories of it. The "repressed memory" idea has largely been discredited as false "memories" brought about by poorly-thought out questioning.

If you have concerns about vague memories when you were a child, etc., be sure to find a therapist who specializes in childhood trauma to help you explore them. People have gone to prison over the false memory issue, so you want to be sure that your recollections are accurate, for your own sake and the sake of the other party as well.