You have a hunger in your heart for Christ.
A candidate for the priesthood finds himself frequently thinking about Jesus and his ministry. There may be moments when your heart overflows with God's love and a deep desire to share this with others. You desire what Christ wanted for the world: the fulfillment of the kingdom of God. You realize that God is calling you to this task.
Other people recognize your vocation and have told you about it.
God works through other people and will often use them to speak to you. If someone has said "You really should be a priest. I can see it in you," or perhaps, "Have you ever thought about becoming a priest? You would make a good one." Other people can often be the first to see a priest's heart in a young man before he can call his own attention to it.
You find yourself attracted to the life and ministry of a priest.
You may find yourself attracted to a specific priest and the way he lives out his priesthood. You may find yourself attracted to the way a priest dresses in "priestly garb." Men being called to the priesthood often find attraction in the work of the priest and his ministry of celebrating Mass, visiting the sick, or the way he preaches on Sunday. These attractions are focused on something much deeper - the ways in which the priest helps his people encounter Jesus in their daily lives. A man who wants to be a priest has a great love for his people and wants them to grow in holiness.
You must continually convince yourself that this is not what God wants from you.
Often when we are fearful and resistant of something we try to convince ourselves that it's not happening. A vocation to the priesthood can cause much fear in one's life. Often men called to the priesthood will spend time trying to talk themselves out of being a priest. If you find yourself doing this it is a strong indication that you need to contact a vocations director for guidance in discerning God's call.