Can Cass predict what your character is thinking/planning/doing? Yes and no. Magneto is fast. Very fast, compared to humans, slow compared to speedsters - and he's always, always plotting. HOWEVER. His eyes? Give him away. Emotions, lies - they are there.
If yes, what limitations do you have on this? His powers - it's a body language in and of itself and it's not something she should be able to figure out easily or quickly. Such as she'll be able to see that he's listening to or looking at something, but not what it IS that's caught his attention or how he's about to use his powers in fine detail.
Is there anything you'd rather she not be able to predict? We'll work that case by case?
Anything else? Am I making sense?