How do you know for sure you are being called into ministry?

Answer

God calls different people to different ministries in different ways. God calls all followers to some sort of ministry, though not necessarily in a pastoral or "official" sense of the term.

Many people simply "know that they know," in ways that many cannot explain, that they are being called. Others can cite signs or reasons that confirmed their calling. Others still sense a calling, only to discover it may not have been real after all.

If you're willing to listen to Him, He'll confirm His calling on you in ways that will be unmistakable to you, though others may argue with it. If you feel God telling you to "go for it," go for it!

Answer

I would recharacterize your question thusly: "Should an uncertain person seek a powerful role?"

The ministry is a very serious business, because of the impact it has on human life, and your question indicates a lack of assuredness. That is, if you were sure of your calling, you would not be asking.

That said, some of the world's most influential people simply found themselves in that position as either a vacuum drew them into it, or their natural ability led them to it. This includes history's tyrants as well, unfortunately. You can wait around for God to smack you with a hammer or just get to work on the task in front of you and see where it leads.