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?
The question of whether you are called to be a priest, or really any other specific calling, is not one you make on your own. If you feel you might be called to such a thing you should follow a number of steps. Since we don't  give personal spiritual advice, this will be generic advice.

First, pray about it. Ask God to show you whether he is really calling you or not. Pray lots, and keep praying during all the remaining stages.

Second, talk to someone about this. Someone who both knows you and knows something about the position you believe you are being called to. Not necessarily someone in that position, but who is not ignorant about it. Alternatively proceed straight to the next step.

Third, talk to your church leader, presumably your own priest. They will obviously know something about the position you think you are called to and a bit about you. They will pray with you and for you, and will give advice and their discernment of the situation. They also know the process, and if they agree that you may be called they will set the next steps in motion.

If after this these people, and you, agree that you might be called to a position then the next stage will be some sort of official discernment process. Part of this process is for the church to decide, with you, whether you are in fact called to a specific role. The details of the process will vary from church to church so there is no point in me describing it. 

Note that this assumes you are in a church with a formalized clergy, which the word 'priest' would seem to imply. If your church is more congregation-based any official process will be very different. But praying about it yourself, and then praying with others, is almost always the best first step.