Sexual abuse has several forms. Even if children or special needs people are "willing," it is sexual abuse because they haven't matured yet.
1. Forcibly touching another person's body using hands and any other part of the body. It's sexual abuse when someone forcibly touches another's butt, crotch, inner thighs, and female chest. This includes intercourse, fondling, oral and anal sex.
2. Forcibly kissing someone.
3. Forcing someone to view sex, masturbation, and sexually explicit material (yes, pornography).
4. Forcing someone to engage in sexual activity with any object or animal. (Includes forcing someone into prostitution.)
5. Taking pictures, videos, and recordings of the naked body and any sexual acts.
6. Obscene conversations over the phone, chatting, in person, or in any other form of communication.
7. Anything else sexual in nature that damages a person emotionally, spiritually, or physically.
A sexual abuser strives to hide evidence of sexual abuse in many ways. The abuser threatens harm or any number of things. A sexual abuser sometimes even appears to be a model citizen.
Get help if you or someone you know has been sexually abused! Unfortunately, you can't always do something because you have a feeling it's happening, but you don't have evidence. I know this feeling well and I wish I could do more. For now, I just try to bring awareness.
I have also written a post on possible signs of a pedophile here.
This link has more detail about signs of sexual abuse and types of sexual abuse.