Sadly many victims don't actually know they are experiencing abuse because the escalation they experience is subtle and happens over time. It is similar to a lobster who slowly gets comfortable with the rising temperature of the water until it dies and begins cooking.
The image for this post is what is called the Power and Control wheel, it is used to help victims identify when they are experiencing abusive situations. Every action on this list is considered an abusive act. Society generally focuses on what Hollywood shows us abuse looks like and that image always involves physical violence in some form.
People don't tend to recognize that abusers are masters at ensuring they are not exposed. The abuser needs to control the victim because they are attempting to feel better themselves. The abuser doesn't go around wanting to hurt anyone, nor does the victim pick a partner hoping they will be beat up. It is an issue that comes out as a result of circumstances, personalities and current stress in the lives of those involved.
It is always better to ask if someone is okay and have the offer to help be turned down than it is to assume that someone doesn't want your involvement.