How do I know that I have a vocation?
A vocation to the priesthood is simply God's Will that you become a priest. The problem arises in knowing God's Will for you in this life. Vocations do not usually happen by visions or interior voices, but rather by signs — signs in your character, piety, and inclinations that the Holy Spirit is moving you to the priesthood. These are the ordinary signs of vocation:
A genuine and constant inclination of mind to serve God as a priest. You feel attracted to the life of the priest, and to ecclesiastical things. You might be interested in the liturgy, sacred dogma, or missionary work. There is something about the priesthood that draws you.
A genuine desire to promote the glory of God, the Church, and the salvation of souls. This is the real work of the priest, and at times demands great sacrifices. This is the only true motive of becoming a priest. It would be wrong to become a priest for the wrong motive, for example, because people would have great respect for you.
A good moral life. This requirement does not mean you must be a saint to consider the priesthood; it simply means that you must be serious about your spiritual life, that you frequent the sacraments often and lead an upright life.
Piety. The life of a priest is a life of prayer, and part of a vocation to the priesthood is an inclination to prayer - liturgical prayer and private prayer.
Emotional stability. The priest must be a parent to all, and must bear the problems of all, and cannot be burdened with emotional and psychological problems.
At least average intelligence. The priest must faithfully transmit the teachings of the Church to the faithful, and accurately diagnose their sins in the confessional. Hence at least an average intellectual ability to pass his seminary courses.
Good physical health. The priest must be in good physical condition in order to carry out the work.
What if I am in doubt about my vocation?
You should go to a good priest whom you know and ask them what they think. Ask the priest to be your regular confessor, and acquaint them candidly with all of your weaknesses and temptations, as well as your strengths and gifts, and trust the priest to counsel you. Even more importantly, pray fervently and perseveringly to God that you be enlightened in this matter.
Should I not wait to go to the seminary until I'm older or have graduated from university?
Absolutely not! A vocation is frequently lost by delay. University will give you only distractions, heavy debts to pay, and courses that will not get you any closer to the priesthood. Better to heed the call as soon as you hear it. Remember, too, that the seminary is where a person tests their vocation by living the clerical life. Whether or not you truly have a vocation will become clear after some time in the seminary.