By developing the argument. The correct answer isn't nearly as important as forming a strong argument, at least in the process of learning, as a well written argument will naturally lead you towards the correct answer.
Its the reason why "showing your work" is taken so strongly. The flaw being that you're not showing your work so much as you're showing your ability to follow a defined series of actions.
The grade school test doesn't ask you to think of a procedure to resolve some stated question. It asks if you already have it memorized, as the formula was handed to you in a prior lecture. Ideally, the test should ask the student to form an argument, thus showing mastery over the knowledge that precedes its creation. This would be judged, and of the students ability to consistently dot the i's and cross his t's, expected but given a minor role (as it is in "real life", mistakes are inevitable and its why we check our work twice, and then give it to others to check again.)
