by theory
With experience, it becomes second nature. On Turn 3, you should be identify at least part of your opponent's hand, and depending on how the shuffles go, you should usually be identify most of your opponent's hand on Turn 7.This is generally not extraordinarily helpful; what is more important is the general principle of anticipating certain cards, particularly scoring cards. And when you are waiting for a particular card and you still haven't seen it by the reshuffle, then you know it's in your opponent's hand.