In the end, fire was the only way to be sure. And if you were wrong... well, there was a reason not many tried for the ashen robes of Master.
Most were content to come to the Citadel, study the six years it took to be considered Educated, and leave to pursue whatever lives required the extra polish. A few, intrigued by the potential of phenomenal cosmic power, continued the additional four years it took to get through the introduction to practical magic. Most of those realized that practical magic was more than taxing enough without getting into the esoteric. But some continued on that path, thinking that perhaps they had what it took, devoting the final five years to finding out.
For every three thousand students that came to the Citadel each year, only three hundred stayed past those initial six years. And of those three hundred, only fifty made it past the tenth year. And of those fifty, perhaps ten saw it through to the end of the fifteenth year. From those ten, perhaps three would have the confidence- in some cases arrogance- to chance the fire. And if you walked into the fire, you would not be permitted to emerge until it died- or you did.
Most died screaming. A few died in dignified silence. And, rarely, one did not die.
That one, when they emerged, was draped in finest silk and called Master, because that is what they were. Masters could bend the fabric of the universe to suit their wills- had to, to survive the fire.
And that is why, in the end, fire was the only way to be sure.