Zenit picks up an article by Tim Drake from the National Catholic Register.
To sum up, Drake's six points:
1) Eucharist, esp. Adoration
2) Invitations by priests to consider the priesthood
3) Faithfulness of the seminary
4) Strong father figure in a candidate's life
5) Interaction with priests, especially as altar servers
6) World Youth Day
They all seem pretty self evident, don't they?