Ordination to the priesthood is always a call and a gift from God. Christ reminded his Apostles that they needed to ask the Lord of the harvest to send laborers into the harvest.  Those who seek priesthood respond generously to God's call using the words of the prophet, "Here I am, send me" (Is 6:8).  This call from God can be recognized and understood from the daily signs that disclose his will to those in charge of discerning the vocation of the candidate.

~from the United States Catholic Catechism for Adults