There is a point in Book 2 where Vincent and Natalia fight (with weapons). She knows when he is about to attack because of a tell. If you’re not familiar with that definition of the word, it’s a poker term meaning to give away the cards you’re holding with some action. Some people become very excited, therefore terrible poker players. Some have small gestures.
Natalia, having been with Vincent for about a year at this point in the trilogy, has noticed something he does right before he moves. It helps her a little.
From Inside the Grey House:
“And how exactly am I doing that?” His voice grew loud. He was feeling a hunger weaving, flowing within him, combining with his anger. He was still under control, but it had taken a lot to heal the last cut. He breathed deep, closed his eyes as Joseph told him his tell.
Natalia waited patiently for Joseph to seal her fate. Joseph most probably knew what she did: that Vincent’s jaw twitched right before he moved. It was something Natalia learned in bed. There were times he would stand unmoving watching her, deciding how to proceed. Every time, without fail, his jaw muscle would twitch right before he dove for her. It was a handy tell to know about in this situation. Natalia felt the slight weight of the rapier, glad now to have the lighter weapon. It would be easier to whip at Vincent than the samurai sword. She stood at the ready as Joseph stepped away.