To share in the mission of Jesus Christ as His priest—to be both priest and victim. To offer the Holy Sacrifice of the Mass and act in the person of Christ. To offer oneself totally for God for the salvation of souls. Are you man enough? No man is perfect–look to the Apostles, the men Jesus called to be the first bishops. The point is Jesus is perfect and that His priesthood lasts forever. This is truly a noble vocation.
How does one know? A man must pray. Spend time in front of the Tabernacle. Pray the Rosary. Go to confession. Talk to a priest and read some good books on the priesthood.
If one has done this and still hears the Lord speaking to the depths of His heart, it is time to take the next step!
Here are some excellent resources for discerning a vocation to the priesthood:
Podcast of To Save a Thousand Souls–an excellent podcast based on the book with the same name. Helpful for discerning diocesan priesthood.
To Save a Thousand Souls–Helpful book which is available for free in most cases.
The Priest is not His Own–A classic book by Fulton Sheen.
Letter to Priests by Benedict XVI–a wonderful reflection on the priesthood and the great model of priests Saint John Vianney.
Sacerdotii Nostri Primordia–Excellent Encyclical written by Pope Saint John XIII on Saint John Vianney