My job has a strong diplomatic and negotiations component. One thing I learned very early in life, is if you want to know what someone is thinking, let them ask you questions. This applies to all negotiations.
If you have the ability to listen, you quickly learn by the areas they want to explore, and what questions they ask, exactly what they're thinking and what their currency is. If they focus on X, or want to know more about Y, you can learn pretty quickly what they care about. I've always called this "reverse interrogation". Let them ask.