By reading what he wrote. DUH?!
That told you how much time he spent doing that thinking? I suspect you might know some of his thoughts by reading his material, but I doubt it can tell you if it was deep thinking or how long he spent arriving at his conclusions. Maybe Reagan was thoughtful enough to think through all of that in ten minutes, or maybe it actually took his entire life.
As for Fred Thompson, I doubt if the absense of written annals are an indication of a lack of thought. Some people write, other don't. That probably has little connection to his leadership potential.