"Am I in God's will?"
There are two ways to answer this question because there are two ways to speak of God's will.
First, there is God's secret will, his will of decree. God's decretive will is that which he has ordained will take place. It includes everything. Because God's secret will includes all that takes place, one cannot be "outside" of God's secret will. So there is a sense in which the answer to the question, "Am I in God's will?" is always, "Yes."
There is, though, another way of speaking of God's will. I am referring to God's revealed or preceptive will. God's revealed will is that which he has commanded us to do in Scripture. The answer to the question, "Am I in God's will?" then depends on whether or not one is acting in obedience to Scripture. When we obey Scripture, we are "in" God's will. When we disobey, we move "outside" of God's (revealed) will for our lives.
God will hold us accountable for living according to his revealed will. His secret will is none of our business (Deuteronomy 29:29). He has told us everything we need to know in order to live a life that is pleasing to him. In those areas where Scripture offers no clear guidance, God has left us free to do as we please. We must, of course, attempt to make decisions in accordance with those things that have been revealed, but sometimes the choice is not always clear. There may, in fact, be two options both perfectly in line with the revealed will of God (Should I take this job or that one? Should I go to this church or that one?).
Ultimately, God's secret will will be done regardless. He will get his way. We should stop asking ourselves, then, about whether this or that action is in accordance with God's secret plan for our lives. This is something we can only know in hindsight. Instead, we should simply focus on obeying that which God has commanded.