You see it as soon as you meet her.
The gnolls were not alone in their fortress. They were too stupid to think of patrols and ambush, too much driven by instinct not to devour their catch immediately, for them to be alone in there. No, the gnolls had been commanded by a handful of men, whom you had seen as they were falling under Minsc's blade, and your own. There were men in that fortress, and Dynaheir stayed there for several days before you found her – alive.
And you know as soon as you meet her, from one woman to another, what it is they did to her.
That night near the campfire, you seek her with your gaze to make sure that she is safe, and she meet it with her own dark eyes of pooled shadows. She does not say anything, but she acknowledges it with her look, that you are right. And then she glances away, at Minsc, and you understand what she is trying to tell you – that she does not want him to know. That he blames himself enough with his failure to avoid her abduction without adding that particular burden to it. Privately, you think that perhaps she does not want him to know of her shame.
Over the days you see her struggling to go back to normal, doing her best not to flinch away from Minsc's spontaneous hugs and pats. And sometimes you see the ranger stealing glances at his witch, pondering and deep unlike anything Minsc normally is. You wonder if, by any chance, he doesn't know. You wonder if these apparent oblivious instances of physical contact aren't his own way of helping his witch reacquaint herself with human touch. And one day as he lays a carefree hand on Dynaheir's arm and does not seem to notice her shifting her weight, he happens to meet your gaze and he gives a small grateful smile. For there is a tacit agreement between the two of you, that you will not tell her of your knowledge.
It is a strange dance you dance, the three of you; the witch who pretends nothing is wrong and indulges the normal behavior of her protector; the ranger who knows and pretends to be oblivious to protect his ward's feelings; both of them knowing that you know but pretending you don't; and you, who plays along both of their tunes.