The thing about any kind of abuse — whether the victim is a child or an adult — is that it affects people differently. This means it is very important not to jump to conclusions based on a superficial reading of the situation. Far too many pop psychology blog posts on this topic make the situation seem simpler than it is, and being labeled a victim when nothing has happened can be quite emotionally stressful for all the people involved.
That said, there are a few textbook signs that may warrant further investigation. Sudden changes in behavior, fear of being touched, fear of being alone with a certain person, self-harm, and substance abuse are all potential signs of abuse. A sudden increase in aggression or anxiety can also be a red flag.
If it turns out that there was abuse taking place, contact the relevant authorities. Depending on the situation, it may also be a good idea to contact an adult abuse lawyer, as this website explains.