The English puritan William Perkins gives us a very helpful answer to this question in his book “The Calling of Ministry.” To know that one is being calling into ministry requires three things: a genuine desire (affirmed by your conscience), tested gifting and learning, and public approval from the church leadership.
Perkins from “The Calling of the Ministry”
“How can you know yourself whether God wants you to go or not? You must ask both your own conscience and the church. For if you are genuinely willing , and are fully and worthily qualified, then God bids you to go. Your conscience must judge of your willingness and the church of your ability. Just as you may not trust other men to judge your inclination or affection, so you may not trust your own judgment to judge your worthiness or adequacy. If therefore your own conscience tells you, after careful self-examination, that you do not love and desire this calling above any other, then God is not sending you. If this is true of you it is not God but some worldly and sinister consideration that has motivated you and put you forward.
But even if you desire the call to ministry, if the church of God does not recognize your sufficiency, God is not sending you. But if, on the contrary, your conscience truly testifies that you desire to serve God and his church in this calling above any other; and if, when you have indicated this to the church and your gifts and learning have been tested, the church (that is, many who are learned, wise, and godly and those whom the church has publicly appointed for that purpose) approves of your desire and of your ability to serve God in this ministry, and if the church issues a public call and bids you go, then God himself has bid you to go. That is as effectual a calling as if you had heard the voice of God from heaven… This is the kind of calling for we are look in these days.
This doctrine justly condemns the presumption of those who act on the basis of private impulses and from carnal motivations. They are justly left without blessing or protection.” (181-182)