How do we know the will of God for our life?

This question is asked often by those seeking to follow Christ faithfully. In short we have to understand the doctrine of God's will. God has a decretive will and a preceptive will. God's decretive will is His secret, unchanging decrees that we only know of when they occur (Ephesians 1:5). God's preceptive will is His commands, precepts, warnings, promises (the canon of Old and New Testament Scripture). In Romans 2:18 Paul outlines God's perceptive will, "18 and know his will and approve what is excellent, because you are instructed from the law".  Notice that Paul connects knowing God's will and approving what is excellent in life with being instructed by the Law of God.

We as God's people do not know God's decretive will unless His preceptive will informs us or as history informs us. For example, we do not know when we will die but we do know that we will die. We do not know what will happen to us in the future but we know the redeemed of God in Christ have a forever home in the presence of the Triune God that awaits us. How do we know who we are to marry (the Word commands us to not be unequally yoked to unbeliever 2 Corinthians 6:14 is the perceptive will of God in this matter)? What job we are to take? What we are to do about this or that? Well, simply put we are live our lives per the promises, commands, and warnings of the Word of God entrusting ourselves to a faithful Creator. We are to daily seek to live out the perceptive will of God (the Word) resting in the reality that the good God who has redeemed us in Christ will guide us and lead us providentially in His decretive (sovereign, unchanging, and secret) will. 

Knowing the will of God for our lives simply means knowing the Word of God and living it out. Having our minds shaped by the Word of God, for in the Word of God we know the will of God. In summary, we have all we need for everyday life and faithfulness to Christ in the Word of God. The question is not "what is God's will for my life?" rather "how does the Word of God say I'm to believe, live, and act?" When we answer the latter the former naturally occurs.

Romans 12:2

2 Do not be conformed to this world, but be transformed by the renewal of your mind, that by testing you may discern what is the will of God, what is good and acceptable and perfect.