You don't even really have to find the "starter circuit relay". If you can access the neutral light (that component should be pretty obvious), you can do your voltage measurements there. The Br wire should have 12V DC on it when the ignition switch is on, and (assuming that the light itself is good) the LT/G wire should have zero volts with the bike in neutral (and the light should illuminate), and the LT/G wire should have 12V DC on it when the bike is not in neutral (and the light should go dark).