In the AG, it seems that "authority" is often obtained by a supernatural experience, known as a "call". I've heard of Bible school students comparing stories of how they got the call. Typically, it's a vision, dream, audible voice, that sort of thing. The more impressive, the better, of course.

But a call isn't the only thing. There are hoops to jump through. Interviews, for example. The powers that be must be convinced that you're for real. (Apparently, it's not that hard to get through it.) Then you get the credentials: either a license or (the next level) ordination.
Tim