The whole of the first four verses of the 16th proverb is about the plans and direction that you are making. This is the way to discern what God wants you to do:
First, listen to what you yourself are saying; it could be God trying to get through.
Second, check your motives. Why are you planning to do a particular thing?
Third, to make sure that you have God's best for you, pray a lot about it and actually go over the whole plan with God.
Fourth, understand your purpose and that God has made everything else for a purpose. You won't find what you are to do until you understand your purpose.
the plans of the heart belong to man
There are deep questions to ponder in this proverb. Is God saying that He allows humans to think their thoughts and have responsibility over their own soul, but He is able to control its output under certain situations?
Is God suggesting that we should make our plans as best we can but pay attention to what we say as the Lord will reveal His will through the things we say?
The word plan is the word preparation or arrangement in the Hebrew. It means "that part of one's life that one plans and seeks to set in order through arrangement." This is a crucial part of living life – making arrangements. It is not more spiritual or godly to just react and go with the natural flow. The natural flow may be sinful.
It is important to realize that this proverb is saying that God does not give us our thoughts to think. We are not puppets on a string in which God is the scriptwriter and the director. The plans that we have actively pursued are ours. There is a strain of Christianity that says that God has predestined everything that everyone will ever think, say, or do. This proverb says no. The thoughts and plans that you have are your own.
but the answer of the tongue is from the Lord
God does speak in those odd-between moments when you speak and out flows some of those things that even you did not expect. It is this ability that God has to interject into your speech – His thoughts and His will – that should keep us paying attention. This kind of thinking is contrary to much of our rationalistic and psychological thinking, but the Scripture declares that God can allow us to understand what He is saying or directing through what we ourselves say. It is amazing.