I was asked to share with you some thoughts on how a person knows they have a vocation to the religious life. It is difficult to pin that down because a vocation is first of all a mystery. God does not just Jump out of the sky and tap us on the shoulder or knock us off a horse the way He did with some people in the Bible. Ordinarily God uses very ordinary instrumental means to let us know what He is calling us to, and that is always a mysterious thing. It is very personal to each one of us. I can't give you any absolute rules for this. I guess, in one sense, the wonderful thing is - that God respects our individuality and treats us all as persons. He respects our freedom and he wants us to use our heads and hearts in trying to discern our vocation. Remember that famous line in A Man For All Seasons, where Thomas More says something like "God made the animals to serve Him by instinct blindly but He made man to serve Him wittingly." That is, to use his wits. We have to use our wits to try to discern what God is asking of us.
To read the whole article: http://www.vocations.com/discern/signs_pable.html