The answer is, most of the time we can’t be certain. I think the problem comes because we use the phrase “God’s will” in a sense different from the biblical writers. We think in terms of decisions, but the will of God in Scripture doesn’t have much to do with decisions about marriage, jobs, money, career, and so on. The will of God in the Bible is usually stated in moral terms:
To become like Christ.
To live a pure life.
To live in harmony with fellow believers.
To pray to the Lord.
To preach the gospel.
To give as God prospers us.
To grow in the Word.
I don’t think God ever intended that we have certainty about this job or that job or living in this city or that city or even marrying this person or that person. We are to seek the Lord, use our intelligence, pray, read the Word, and then trust God to guide our steps as we move forward.
I believe the whole search for “certainty” is misguided in that it seeks something God never promised. That’s why the Bible says we walk by faith, not by sight.
On many decisions of life, I think we are intended to live in the “perhaps” realm because, frankly, those things we tend to obsess over really won’t matter that much 10,000 years from now. But what will matter is whether or not we truly sought to be godly, to walk in the Spirit, to manifest the life of Christ, and to make the gospel beautiful by the way we live. Those things are truly the will of God for all of us all the time.
That said, I think the “certainty” we seek will often come after a decision and not before. Many times we must take a step of faith, trusting God to lead us. Only later can we look back and that God was involved even when we weren’t certain about what we were doing. See When You Need to Know, You’ll Know.