How is a sub supposed to know when they're being abused, not dominated? Can they take control back to fix the situation or is that something the dom controls?
Having never really faced this situation, I had to get thinking a lot.
What I am gonna say will be controversial and misunderstood, but some people are in abusive situations and they are happy about it.
Some subs might consider themselves slaves and enjoy being beaten up by their masters. Some subs are actually quite assertive and "dominant" in their everyday lives and need a Dom that understand that once play time is over, they go back to being partners.
That last part is something I always had trouble with. I am a type A personality: I control everything and anything, at all time. Except in bed. That's how I get my fun, I disconnect, I become a fuck puppet. Someone's toy to enjoy. I would quickly see the difference between being abused and dominated cause I do no let the submission out of the bedroom.