How does one know if God is calling him to be a priest?

**According to Catholicism, How does one know if God is calling him to be a priest?** If one is contemplating being a diocesan priest, what is the normal way one should do things in order to determine his discernment towards this call? Is there a different approach taken for those who are considered late vocations? For example take my own circumstances: Earlier in my life, I ran after the highest level of education and tried very hard to achieve it. But after 20 years of effort, I obtained a certificate. I ran after every secular thing that there is. Nothing seems to attract me. So I am just wondering how does one knows if God is calling him to be a priest and what steps does one normally take to find out in the Catholic Church? What should one do?