70 The Head of the seminary lovingly follows the youths entrusted to his care and studies the inclinations of each. His watchful and experienced eye will perceive, without difficulty, whether one or other have, or have not, a true priestly vocation. This, as you well know, Venerable Brethren, is not established so much by some inner feeling or devout attraction, which may sometimes be absent or hardly perceptible; but rather by a right intention in the aspirant together with a combination of physical, intellectual and moral qualities which make him fitted for such a state of life. He must look to the priesthood solely from the noble motive of consecrating himself to the service of God and the salvation of souls; he must likewise have, or at least strive earnestly to acquire, solid piety, perfect purity of life and sufficient knowledge such as We have explained on a previous page. Thus he shows that he is called by God to the priestly state. Whoever, on the other hand, urged on, perhaps, by ill-advised parents, looks to this state as a means to temporal and earthly gains which he imagines and desires in the priesthood, as happened more often in the past; whoever is intractable, unruly or undisciplined, has small taste for piety, is not industrious, and shows little zeal for souls; whoever has a special tendency to sensuality, and after long trial has not proved he can conquer it; whoever has no aptitude for study and who will be unable to follow the prescribed courses with due satisfaction; all such cases show that they are not intended for the priesthood. By letting them go on almost to the threshold of the sanctuary, superiors only make it ever more difficult for them to draw back; and, perhaps, even cause them to accept ordination through human respect, without vocation and without the priestly spirit.
Source: Ad Catholici Sacerdotii (Vatican.va)