There are several tools God has given to help determine the calling God has given us. The first is our life history, our life story.
Let's pretend for a moment that there is an infinite, personal God, who made man-kind, designed them to have a relationship with him, and designed everything about individual people ... including what will enable them to live to their greatest potential. And since he is an infinite, personal God, he can give each of his children moment by moment attention. He can tailor a detailed program of training for each child, so they fill that spot that we have given the term "calling:" fulfilling their potential, acting in the most effective and efficient way to complete God's purpose and mission, maximizing joy and giving love in an individual and unique way.
If there was such a God, and he had such a purpose for his children, we should be able to see his hand in their lives. There should be lessons, themes and patterns. There should be experiences that unlock doors to unseen, unknown vistas. There should be principles that are etched indelibly on hearts.
Now let's pretend there is an enemy. One who want to detract from God's glory, thwart his purposes, disrupt his mission and generate as much misery as possible. And since God's glory, purposes and mission are bound up in people, (people choosing to align themselves with God in a new allegiance, to live in a new life that points others to God and to partner with God in restoring his kingdom) the place the enemy will attack is people ... especially his children. He will try to disrupt their training, distract them filling their place, confuse their identity, and interfere with their relationship with the Father.
If a person charted his life, there would be a number of ups and downs. And a number of these ups and downs would affect decisions, changing the direction of life. Some decisions would be based on fear. Some on faith, love or any of a number of practical considerations. By charting these transitions, a person could begin (repeat: begin) to see themes and patterns. One might also see places of disruption. To see the place of calling, one needs a panoramic view. There are motivations, and contexts. So, a person's life story is a view from one mountain top.