You'll just know! It will be the answer that fills you with the most emotion. You may even cry. The idea will resonate with you completely, and there won't be any doubt. Just write down whatever comes into your mind until you reach that point.
Commonly, answers will repeat themselves. A lot of unrelated nonsense is also likely to come out. Our brains can be a little cluttered, and this exercise will expose that fact. Don't let anything you write distract you from your intentions; odd things quite frequently end up on the paper.
There will be several answers along the path that feel pretty good to you, but remember, you're looking for the big one. When you see the ones that are good, but not "it," this means you're close, but not quite on target yet.
You're looking for that one idea that feels overwhelming to you. In this exercise, it's common for your life purpose to reveal itself in less than 100 ideas, but it can take as many as 400. Continue writing!