That comes when you pay attention to how you use brushes as you work. You'll find that your rational selection procedures don't always serve your instinctive reach for one brush rather than another. In some cases these instincts are just habits that new brushes will allow you to break. In other cases they reflect the goals of your painting and are fundamental to your style or working tactics. You'll do this with the pleasing awareness that your choices depend as much on your capabilities and desires as on the objective structure of your tools.
Art is as much about self-discovery as skill.
You can buy the best brushes on the market, but if they don't behave in the way you want and are comfortable with, you'll be unhappy.