Please don't understand this as an attack or anything like that, but I really feel that I have to comment on that:
I agree that in the worst cases you can't tell. I'd even say that in most cases you can't tell -- and that in most cases, victims of abuse of any kind will deny being abused. It is usually a very long and very painful journey to figure out that abuse is abuse and not "him being difficult".
What I would strongly like to contest is the notion that "These will be people who think badly of themselves and are generally not comfortable with who they are." Not all people who have issues with themselves are in abusive relationships. Neither is having issues an open invitation for abuse. This is completely disregarding just how skillful abusers are: They are masters of manipulation and deception. They will, in most cases, appear to you as the most wonderful persons you have ever met...kind, supportive, caring -- you name it, they do it. They establish themselves in a certain way, and behind the facade start to undermine their victims' self-esteem, as well as their conviction or even the slightest thought that they have some value at all.
I would also like to add that I find a phrase like "they will find someone to hurt 'em eventually" offensive, although I am sure it was not meant that way. This is blaming the victims of abuse for the abuse they had or have to endure. People who are abused do not run around searching for somebody to hurt them. They, at least in most cases, fall in love with somebody who will not treat them like they deserve. They lack the healthy reaction of saying "enough". They believe that the way they are treated is normal, that this is how life works, and often have never experienced anything different either. It is not wrong to believe that if you tell your partner that you are hurt by something, it will be in their best interest to stop it. This is not the same as actively finding somebody to hurt you.
I really don't want to write a novel about the dynamics of abusive relationships, I just felt the need to clarify that. I don't know how to address the original problem in this thread effectively...it is very hard to tell from the outside, and the power gradient that signifies many BDSM relationships is, I believe, further disguising the problematic (I do not mean that BDSM and abuse of any kind have anything in common, just in case this came out wrong). If you were on the receiving end, I'd say "trust your guts" -- if it feels like abuse, it is abuse. I am not sure if this is also applicable to an outside observer.
In general, I believe that there are several signs one can watch out for. Disrespect would be one. Abusers -- I am generalising here -- respect nobody but themselves, and least of all their partners. So if there is no respect, no caring towards the other I'd say it's an indicator. Another thing would be 'irrational' emotional responses, most of all fear, expressed in situations that don't really call for such a reaction. I'd say watch out for the subtle things, and talk to the maybe-victim if you feel something is odd. I'm afraid that doesn't help a lot after all. As said, I don't know how to solve the problem.
Asides that, it is NOT TRUE that you cannot help victims of abuse. You cannot force them to leave their abusers, and you cannot force them to never step into the same traps again. But you can point them into the right direction, be understanding and supportive -- you can help to lift the veil. Most of us don't want to see the light, simply because looking at it means to look at your world collapse. At the same time, once you see it -- well, you can't look away anymore. And that's the mother of all change.
I'm sorry if this didn't help much, it's just a topic that I find personally and universally important. So, just my two cents coming from my own and subjective perception![]()