Admittedly, it is hard to tell. No one knows for certain without a lot of prayer, reflection, and some good discernment, which includes talking with others you know and trust. There are some signs that are good benchmarks. If you agree with most of the statements below, you should talk with a vocation director or seek some spiritual guidance.
1. Other people tell you so.
2. Your prayer life is getting better.
3. You are attending Mass and the sacraments more frequently.
4. You are spending more time in church and/or service-related activities.
5. You progress beyond a generic idea of being a priest to imagining yourself acting as one more concretely, e.g., preaching, teaching, celebrating the sacraments.
6. You find yourself fighting it but can’t get it out of your head.
7. You have other good options but wonder if you will regret it some day if you don’t give the seminary a try.
What can I do to test my vocation if I’m not sure?
1. Pray, especially in front of the Blessed Sacrament.
2. Go to daily Mass wherever possible.
3. Volunteer to get more involved in parish or campus ministry activities. Do you enjoy it? Is the thought of doing ministry for a lifetime increasingly appealing?
4. Seek guidance from a spiritual director, a priest, brother, or sister, with whom you can discuss your spiritual journey. Discuss it with your family and friends who know you best.
5. Try other things. Pursue jobs or internships in careers that appeal to you. If you’re thinking about applying for a job or for graduate school or for a job, can you write a convincing resume or essay about why you want to do those things that sounds convincing to you? Is your heart pulled more toward these things or toward serving God as a brother or priest?
No one is ever worthy, but a person who feels called to enter a seminary should take that very seriously, discuss it with those close to him, seek spiritual advice from a priest, and pray over the matter with earnestness. Rather than just thinking about it, bring it to those you trust.