How do we get to know our spouse's or good friend's interests, likes and dislikes, idiosyncrasies? By spending lots of time with them, of course!
Dr. Sweeting points out that we get to know God's will in the same way. We do that in prayer and meditation on God's Word--lots of it. The closer we commune with our Heavenly Father, the better we are going to know Him. The more time that we spend in His presence, the less problem we will have in determining His will.